welcome to bbc news, broadcasting to viewers in north america and around the globe. our top stories: a wave of toxic destruction. dramatic new footage shows the momenta dam collapsed in brazil. at least 110 people are dead and more than 200 still missing. the question that everyone here isjust stunned by is how, in a big, modern, growing economy, this could ever have been allowed to happen. russia condemns america's decision to pull out of a nuclear disarmament treaty signed during the cold war. the us says moscow's new cruise missile violates the terms. we have a special report from venezuela, where opponents of nicolas maduro say they have been detained and tortured in the past few days. and want to feel better? a new study suggests that staying off social media use will do you a world of good. dramatic pictures have emerged of the moment a dam burst in brazil,

releasing millions of tonnes of mining waste that engulfed nearby buildings. at least 110 people are now known to have died in the disaster, which happened in the state of minas gerais last month. at least 200 are still missing. our science editor david shukman reports from the site of the dam, in the south—east of brazil. first, a long cloud of dust, then a nightmare vision of an unrelenting torrent of sludge, the waste from decades of mining racing towards hundreds of unsuspecting people down below. the catastrophe unfolded a week ago, but only now has this video come to light, adding to the sense of loss and of outrage. and difficult for you. this red cross volunteer, henato silveira, leads me to the edge of the disaster zone. seis corpos.

six bodies. he alone has found six bodies. any hope of reaching survivors in this endless sea of mud was quickly dashed. emergency workers are now scouting for any signs of bodies from the air, and they are picking their way over this horrific landscape. we spotted this search team with a sniffer dog in the distance. by the time the wall of mud reached this point, it had already overwhelmed the cafeteria where the miners were having lunch and destroyed the offices of the mine itself, before arriving here, tearing through a hotel and holiday chalets, before surging on over that ridge in the distance and down into the valley beyond, where it caused yet more destruction. and the question that everyone here isjust stunned by is how, in a big, modern, growing economy, this could ever have been allowed to happen. investigators are now on the scene. the dam holding back the waste

was owned by one of the world's largest mining companies. it was inspected only last year. we find a local man, leandro gil, praying for friends lost in the mud. "unfortu nately", he says, "someone just thought about himself. he didn't protect the dam properly. so, after years and years, that's now been revealed." a special mass, seven days since the disaster. there is grief and anger, and the demand for answers will only grow. david shukman, bbc news, in brazil. for more on the situation in brazil, head to the bbc news website. along with more on the search for the missing, we are asking the question, how do you get rid of tonnes of toxic waste? find the answer by going to bbc.com/news, and then follow the links. could the world be on the verge of a new nuclear arms race between the us and russia? that is the question being asked,

after the us suspended a landmark deal which has been in force for more than three decades. it is accusing russia of breaking it, something russia denies. the agreement in question outlaws the use of intermediate—range nuclear missiles stationed on land. laura westbrook reports. in 1987, us president ronald reagan and soviet leader mikhail gorbachev signed the intermediate—range nuclear forces treaty. it led to hundreds of missiles in both countries being destroyed. three decades later, the us secretary of state made this announcement. russia has jeopardised the united states‘ security interests, and we can no longer be restricted by the treaty while russia shamelessly violates it. nato agrees with the decision by the united states. we continue to call on russia to come back into compliance in a verifiable way, because we don't want a new arms race. this wasn't unexpected.

for years, america has been concerned about russia's testing and deployment of a cruise missile that it says breaches the inf treaty. there's concerns other countries, including china, aren't tied by the accord. for now, the treaty is suspended. the us could pull out formally in six months, but gave russia a final chance to save the deal. i hope that we're able to get everybody in a very big and beautiful room, and do a new treaty that would be much better, but — ‘cause certainly i would like to see that. for its part, the kremlin says this was a political decision. russia denies the missile in question is in violation of the treaty. this was iran's response. the foreign minister tweeted. .. this was the image that let

the world know the cold war was coming to an end. 30 years later, many are concerned about the future of nuclear arms control. laura westbrook, bbc news. earlier i spoke to dr michael carpenter. he is a former us deputy assistant secretary of defense, and he was also director for russia at the us national security council in the obama administration. i asked him if the treaty was dead, and if so, why now. for all intents and purposes, it is dead. i think it was inevitable that at some point the us would withdraw from the treaty because of russia's cheating, essentially, in both development and then operational deployment of this treaty—prohibited missile. the question that you raise, though, is the important one — why now? and there really is no good answer to that question. the united states should, frankly, have waited until it was ready with its own intermediate—range missile to be able to withdraw from the treaty.

instead, precipitously, right the trump administration is saying it is going to withdraw. there is absolutely no reason for it to do so, and it leads some nations to question whether in fact the united states is the one that is being honest with the international community, as opposed to russia, which is in fact cheating and therefore necessitates this breach, butjust not right now. the us has suspended its part in the treaty, pending perhaps any change on russia's side. but hasn't the us also violated this treaty in the past? there are both parties here. no, so the united states has not violated this treaty in the past. the treaty has held largely intact until about three or four years ago, when russia started the r&d programme and the development of this missile, which has a range of between 500 and 5,500 kilometres, which is the proscribed range according to the treaty. so the russians are the one who violated it. we have very good intelligence that we shared with our allies

to prove that this was in fact the case. but, again, the precipitous withdrawal right now raises questions as to why the united states needed to do this. why couldn't we have given more time to negotiations? why wouldn't we have at least we waited until we were ready with our own missile, as opposed tojumping the gun and looking like the fall guy to a large part of our allies, friends and partners? it's a good question. so what do you think is really going on, then? well, i think the trump administration wanted to prove that it was going to be tough. i think a lot of people in this administration don't believe in arms control, and so they saw this as a way to telegraph that to their base, that we were withdrawing from a treaty, that we weren't going to allow the russians to play us for fools, that we were going to sort of stand up to them. i think those sorts of considerations played into this, but as a strategic choice, it's a very poor one.

the democratic governor of the us state of virginia has apologised for appearing in a racist and offensive photograph in his medical school yearbook from 1984. the republican party in virginia has called for ralph northam to resign, saying there was no place for racism in the state. that demand was echoed by some democrats. in a video posted on twitter, ralph northam did not say if he was the man appearing in blackface or the one dressed in ku klux klan robes in the picture. but he expressed his remorse, while resisting calls to quit. that photo, and the racist and offensive attitudes it represents, does not reflect that person i am today, all the way that i have conducted myself as a soldier, a doctor and a public servant. i am deeply sorry. i cannot change the decisions i made, nor can they

understand the harm my behaviour caused and today. but i accept responsibility for my past actions, andi responsibility for my past actions, and i am ready to do the hard work of regaining your trust. venezuela's opposition leader, juan guaido, is calling for the biggest mass protests in the nation's history this weekend as he tries to force president nicolas maduro from power. the opposition are demanding new elections to end the political and economic chaos engulfing the country. human rights lawyers have told the bbc there has been a wave of political arrests, with almost 1,000 people detained in recent days. the government denies it tortures prisoners, but our international correspondent 0rla guerin has heard a harrowing account from one women who says she was beaten and abused. you may find some of 0rla's report upsetting. under darkened skies, caracas waits, some here hoping for a modern—day liberator to replace president nicolas maduro. but those who dare to oppose him can expect to pay a price.

almost 1,000 people have been detained in the past ten days. we're on our way now to meet one of them. she is a young woman who we can't identify for her own safety. she has just been released, and she has a harrowing story to tell. she says she was held in this building, the headquarters of the military police. they interrogated her about a relative, a military officer suspected of plotting against the president. translation: they tortured me. they put a plastic bag over my face and choked me. then they put my head in a bucket of water to try to drown me. i fainted, and they beat me to wake me up. they put a gun in my mouth and cocked it. they said they wanted answers fast, and that they loved

to see people suffer. their boss told them all to rape me. they lifted my shirt and took off my bra and they touched me. did you think that you might be killed? translation: yes, because they were constantly saying it. they said they were going to kill me and throw me in the river. they said, "we are the government, and nobody can do anything against the government". and leading human rights lawyer alfredo romero says venezuela's government is in overdrive, trying to silence dissent. now we can talk also about generalised persecution, generalised detentions, that they detained anyone in order to intimidate people and produce what they are producing — this fear of protesting. and that's what is going on. but some, like federica romer,

a politics student, are determined to keep up the fight. she says being a mother to six—month—old carlota means she has to protest. this is for her, you know? this is for her. before i had her, ifelt invincible and i did not care. and now that i have her, of course, i'm scared to go out. i'm terrified. but this is absolutely for her, you know? ‘cause she is the next generation of this country, and she... they‘re the ones who are going to hold this country up. federica is preparing to go back out on the streets tomorrow at a mass demonstration called by the opposition, gas mask at the ready. 0rla guerin, bbc news, caracas. stay with us on bbc news. still to come: a0 years

after ayatollah khomeini returned to iran, we look back at the day that changed history. this is the moment that millions in iran had been waiting for. after his long years in exile, the first hesitant steps of ayatollah khomeini on iranian soil. south africa‘s white government has offered its black opponents concessions unparalleled in the history of apartheid. and the anc leader, nelson mandela, is to be set free unconditionally. ..four, three, two, one... a countdown to a critical moment. the world's most powerful rocket ignited all 27 of its engines at once. and, apart from its power, it's this recycling of the rocket, slashing the cost of a launch, that makes this a breakthrough in the business of space travel. two americans have become the first humans to walk in space without any lifeline to their spaceship. one of them called it a piece of cake. thousands of people have given

the yachtswoman ellen macarthur a spectacular homecoming in the cornish port of falmouth, after she smashed the world record for sailing solo around the world non—stop. this is bbc world news. the latest headlines: dramatic new footage has emerged showing the moment a dam collapsed in brazil. at least 110 people are confirmed dead. the united states suspends a major cold war arms agreement with russia, saying it has violated the treaty with a new missile system. a0 years ago, the exiled cleric ayatollah khomeini, returned to his native iran helping to establish a revolution that recast the country and its role in the middle east. iran‘s ties with the west were radically changed, and the ayatollah became the supreme leader of the new islamic republic of iran. our world affairs editorjohn simpson was on that flight

from paris to tehran with the ayatollah, and witnessed the revolution unfold. it was a key moment in modern history. underthe shah, iran was increasingly westernised, but its vast oil wealth brought corruption and social upheaval and that led to a year of violent demonstrations. ayatollah khomeini was in exile in france. he was implacable. the shah must go. and soon, the shah was forced to escape abroad, a broken man, dying of cancer. throughout the revolution, sir david reddaway was a young diplomat in the british embassy. when we were in the middle of all the events of the collapse of the old iran, neither we nor really anybody else knew what was going to come out of it.

and i don‘t think many iranians knew what they would be seeing over this timescale. 0n khomeini‘s flight back to iran, i asked him what he was feeling. he ignored me. a moment later, someone else asked the same question. "nothing", he said. his return was a huge defeat for the west and a massive boost to islamic fundamentalism. even today, the pictures are still extraordinary. these were the crowds outside. i mean, people say it was the biggest crowd in human history. it was certainly a couple of million, maybe 3 million. there was a brief showdown between the army and the demonstrators a few days later. i watched as the victorious crowds trashed a secret police headquarters. archive: the policeman‘s uniforms and even the files that they have found inside have been scattered to the winds and the demonstrators are in complete possession. ever since, the clerics have ruled

iran, clamping down in particular on women‘s rights. the only real challenge for them came in 2009 after a disputed presidential election. it was called the green revolution and it rocked the religious leadership, but it was put down savagely. the bbc‘s persian service is banned in iran, but millions watch it anyway. rana rahimpour is one of its presenters. there is appetite for a change, but because there is nobody to mobilise the youth who want change, i cannot imagine another revolution happening, despite the widespread corruption. its economy is troubled, but modern iran can be surprisingly relaxed and pleasant. still, people are scared to come out against the government. that has kept the system going for a0 years. john simpson, bbc news. it‘s feared the death toll linked

to the us deep freeze may rise. at least 21 people have died during the cold snap. temperatures fell to —30 degrees celsius in places like chicago. with with more milder conditions forecast, emergency services say melting ice and snow could increase the chance of flooding and mudslides. new research out of stanford and new york university takes a look at what happens when people step back from facebook for a month. the study, entitled the welfare effects of social media, finds that disengaging from facebook leads to small but significant improvements in wellbeing. earlier i spoke with jaron laneer, a computer scientist and silicon valley insider. he‘s also the author of "10 arguments for deleting your social media accounts right now". i began by asking what he made of the study. it appears to be well done study, and the results are consistent with other research, although they are so few new questions that have not been studied this formally before.

the picture that has emerged is that when people leave social media as it is, meaning the existing social media companies like facebook, they experience benefits. a really interesting thing in this study is that their particular population found itself less well informed after they quit, but i would attribute that to not having ultimate skills being informed. there are other studies showing people becoming that are informed after leaving the current social media environment. another wonderful thing about the study is they documented the positive effects which are intrinsic to the internet, that people also gained the social benefits that are obviously present, which we know intuitively, in this case is shown. it highlights the tragedy that something that should he so intrinsically beneficial has this added layer of negativity on it, because of the nature of the current commercial companies that control social media. and itjust reinforces i think the argument that we need to have changes.

we are hearing more and more about the negatives of social media, specifically with facebook so, given what has come out of this study, can facebook change? is it possible for those negative element to be negated somehow? you know... i am absolutely convinced that facebook can change its business model and become more like netflix, where those who can afford to pay it, you get ptv with net flicks, it you get peak social media with facebook. just because i am convinced does not mean mr zuckerberg is convinced. facebook is an entirely unusual corporation in that it really is controlled by a single person. he has the majority of the voting stock. it is barely a corporation according to our usual understanding, it is one person. and that gives me optimism, i believe he will come to see

the light, that he can have an excellent business, be a better business that is also better for the world, if he gets rid of this massive manipulation machine which is currently the way they make money. when you run a business, every penny you make is because somebody thought they could manipulate someone else. of course what you will get is a twisted mess. when you run a business for people who paper what they get, you get capitalism, and can cripple as capitalism, —— criticise capitalism, but it is not as twisty and screwy as the current advertising model is. are some of the criticisms of facebook a little bit unfair? surely if you use it in moderation, like with everything else, you get the benefits, you are able to stay connected, you are able to organise your social life, but not wholly live within the app? i think the best answer comes from

facebook‘s own researchers, who have published peer—reviewed papers where they proved they could make people sad deliberately without the people being aware of it. that latter part is the key. it is really not humanly possible to be aware of what is being done to you when you are engaging four hours a day with this system, that is solely making money by manipulating you. people get tricky, it gets more and more sophisticated. if it were possible to be aware of what is being done to you that i would agree with that completely. unfortunately it has proven to not be possible, thus we must criticise facebook. it‘s only a matter of days now until the lunar new year. people in china, korea, vietnam and many other nations in asia will be celebrating. in fact — for some — the festivities are already underway. as are the journeys many are taking to be with their loved ones. the bbc‘s tim allman has more. it looks like all of china is on the move. whether it is by train...

0r plane... or even by ship. more than 500 million trips have already been made, nearly 3 billion are expected to be completed before the celebrations come to a close. this man is heading home to sichuan province, a journey that is expected to last around 30 hours. this woman is taking her a—year—old daughter back to anhui province, which is a snip at only 20 hours. the lunar new year, or spring festival, is associated with light and colour. and if that is what you‘re after, head to guiyang city, where more than 1000 lanterns illuminate the night sky. lights of a different kind in the north of

china, molten iron thrown into the air. all the while, dragon dancing takes place as the redhot metal showers to the ground. translation: it is really amazing when the molten iron is all through the sky. translation: it is really amazing when the molten iron is all through the sky. translation: it is breathtaking seeing people performing the dragon dance against the backdrop of the molten iron sparkles. even these panda cubs were getting into the swing of it at this nature reserve. this will be the year of the pig, so they got to play with some soft cuddly toy versions. no contest as to who was cuter though. happy lunar new year to you too. you can reach me on twitter — i‘m @regedahmadbbc. hello.

friday was another day with severe transport disruption due to heavy snow. it was across central and southern england that had the worst of it, particularly stretches of the m3 towards the basingstoke area. we also had troubles around the m2 in kent as well late in the day. some of those problems were pictured from the greywell area overlooking the m3, you can see how horrendous that conditions look there on the main carriageway, completely covered in snow. transport disruption has been a factor with the forecast through friday, and again into saturday too, with the risk of some snow still around, but ice as well fairly widely. looking at the weather picture over the next few hours, the snow that we have across south—east england becoming confined to kent before slowly easing away. there will be wintry showers across the eastern side of scotland into eastern parts of england, wherever those showers fall of course that adds to the risk of ice with a widespread and sharp frost once again.

the forecast for saturday is that yes, there will be more wintry weather around, and after a cold and frosty start, icy too, and we will see wintry showers trickling down the eastern coast, probably saying quite cloudy for a good part of the morning across eastern scotland, maybe with some flurries, but nothing too heavy. chancew of a few showers affecting western wales but they will ease with time. we may see a few showers coming in across the north—west of northern ireland into western scotland. again, showers are a bit of a mixture here with some rain around sea level, we could see some sleet and snow mixed in. saturday, a decent day with some bright or sunny spells after that cold and icy start. temperatures will be struggling once again, looking at another cold day for the time of year. mostly around 3—a celsius. that goes into saturday night with clear skies leading to a sharp fall in temperatures for the most of the uk. —6 in newcastle, there is the risk of some icy stretches, but further west we will see some cloud thickened as the weather system approaches of the atlantic for sunday. this is going to bring another

spell of wintry weather. we could see a few more centimetres of snow, targeting possibly the high ground in northern ireland, but the far north of england and scotland could see maybe 3—6 centimetres, but there is a tendency at lower levels for that snow to transition back to rain as mild air tries to move in from the west. 5—6 for glasgow and belfast. into next week it is all changing, it turns more mild but there will be some heavy rain around as well and it will often be windy. this is bbc news. the headlines: dramatic pictures have emerged of the moment a dam burst in brazil, releasing millions of tonnes of mining waste that engulfed nearby buildings. more than 100 people are now known to have died in the disaster, which happened in the state of minas gerais last month. russia has called america‘s decision

to suspend its participation in a key cold war—era disarmament agreement an abrogation of its international commitments. a russian official said washington had provided no evidence of its claim that russia had for years failed to comply with the intermediate—range nuclear forces treaty. venezuela‘s opposition leader, juan guaido, is calling for the biggest mass protests in the nation‘s history this weekend, as he tries to force president nicolas maduro from power. the opposition are demanding new elections to end the political and economic chaos that has been engulfing the country. tributes have been paid to the comedianjeremy hardy, who has died at the age of 57. he had been receving treatment for cancer.