come back to save. get ten dollars off with no exclusions! we make style affordable, you make it yours! jcpenney. tonight, on "nightline," premeditated mayhem. as darkness false on one american city tonight, police fight back against flash mobs gone bad. angry crowds of kids on the rampage. we're on the scene. zbrnlgts fire in the sky. a celestial mystery. tens of thousands of strike almost every night. why does it happen in. and, she's back. while republicans duke it out in iowa, sarah palin drops in and siphons off the love. is she running for president or

does she want even more? we're in the heartland. we ask her. good friday evening. i'm bill weir. tonight, the city of philadelphia is saying enough to a cycle of violence driven by crowds of young teenage irs using social media to link up, form a mass and cause chaos. it's a sinister version of flash mobs. for some reason, the city of brotherlrl love, the term flash mob means loosely organized violence. >> reporter: maybe it's summer. but the senseless violence of flash mobs set london on fire this week. philadelphia is not lopden. it's not burning buildings. t the episodes of violence

like these have been going on for months. we see kids intent on stealing, on scaring passers by. on trying to have a night out. >> there's no real pattern to it. but it's very troubling. we have had people get serious injuries. >> reporter: so today, as evening approached in philadelphia, a new measure was planned. children under 18, teens like these, were told that as of 9:00 p.m., they had to be off the streets downtown. >> we have had problems with kids gathering in groups for decades. what ps n what's new is the use of social media. at 9:00 p.m. approachedtonight, it was clear there's serious. this is philadelphia's mayor. >> if you want to act like a butthead, your butt is going to get locked up. if you're going act like an

idiot, move. we don't want you here no more. >> reporter: you don't hear talk like this often. >> this nonsense must stop. >> reporter: it's not just the kids, the parents need the get it under control. >> parents who don't know where their kids are, you'll find yourselves spending quality time with your kids in jail. >> reporter: this is las vegas. a mob rushing a convenience store to carry away the contents. this is a clothing store in washington, d.c. it's amazing to see how nonchalantly they're going about their work. >> they thought it was funny. but we sure don't. >> reporter: flash mobs didn't start out this way. they used to be a fun stunt. a group like this signaled by a twitter message or facebook posting, they would dance or just freeze and leave.

but then the idea got hijacked. >> a whole mind set and culture where these random assaults are fun or that you should video tape them and put them on youtube. >> a lack of reflection on consequences. somebody does it. pushes over the lamp post, that almost validates it maybe for you to do it. >> reporter: that's what seems to be in play a few days ago at the wisconsin state fair. the local 9 is 1 call center lit up. >> my millimeter just got attacked by a flash mob. >> my friend got hit in the face. >> reporter: the major and others in philadelphia are making race part of the conversation. >> you damage yourself. you're damaging another person. you damage your pierce. and your damage your own race. >> reporter: edward montgomery, father of five, says it's not

about race. >> who is supervising the children? it's a lot of broken homes. that's -- you know, tearing up children on the inside. >> reporter: as an alternative outside the curfew zone, rec centers like this are kept open. early this evening, kids that don't get into the violence made predictions. >> i think kids will stay out. they don't care. >> kids, not listening to the parents, they just don't care. >> reporter: and so 9:00 p.m. came. was real trouble expected? not necessarily. were any kids expected to break curfew? here's the deal of the commissioner. >> we have contacted the parents. they'll be written up. the parents will have to come to the police station to get them. you'll always have a few knuckle heads that don't pay attention. >> reporter: which is why the van was filling up.

some said they were unaware of the curfew. going alalg peacefully. that taps system now. itit was meant to keep going li that all the way through until morning. i'm john donvan, for "nightline." the natives call it the river of fire in the sky. we cut through the young toll get front row seats for the greatest lightning show on earth. yeah. 24 bucks later. that hurts. it's not like i really had a choice. snack on this. progressive's "name your price" tool showed me a rangngof coverages and i picked the one that worked for me. i saved hundreds. wow, that's dinner and a movie. [ dramatic soundtrack plays ] this picture stars you and savings. but mostly savings. out there with a better way. now, that's progressive.

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spend some time in oklahoma as a kid, where watching electrical storms from the porch beat anything on tv. that lightning was small bore

compared to a corner of venezuela where the rate of strikes is higher than anywhere else on earth. natives took the sky for granted until one night the show stopped. here's abc's matt gutman. >> reporter: it's a symphony of night. the cease light up the nighttime sky. bolts crash. 40,000 of them. rapid fire. up to 300 nights a year. the greatest night shows on earth. people here called this rebaba, the river of fire in the sky. it was as reliable as yellowstone's old faithful. until it wasn't. last year, the lightning disappeared. and no one knew why. our journey to explore this

mystery begins with a drive through scrub brush to lake maracaibo, the largest in south america. time enough to get to know eric. probably the world's most foremostst expert on the river fire. >> translator: i saw my first lightning when i was 8 years old. >> the lightning to them is like cars on the street in new york city. >> reporter: and then there's alan. he takes museum quality photographs of the storms and lives part-time on the lake with the villagers. >> i try to put it into their minds that's something special. unique in the world. >> reporter: but to get to alan's hut, you have to motor out to the tropical jungle. once on the lake we pick up dinner. soft-shelled crabs. wow.

just 25 cents a pound. and then these stilt villages, built generation ago, for a life on the water. no cars. no bicycles. just boats. for the villagers here in a place called congo, the lightning just became part of their lives. >> translator: there were some visitors that came here and spent the whole night on the water looking at lightning. >> reporter: did you think they were crazy in. >> translator: yeah, maybe. who will stay up watching stuff like that? >> reporter: but folks like the village elder wrote poems about it. >> translator: when the sun sets, the spirit of catatombo lights up the sky. >> reporter: what did you think when it stopped? >> translator: we did realize the lightning stopped.

ititas mysterious. we don't have the informamaon to say exactly why. >> reporter: he thinks it may have something to do with a global weather pattern called la nina. it may be tied to the freakish weather elsewhere. >> oh, gosh, that is a monster tornado. >> reporter: the unusually deadly tornados that ripped through the u.s. the droughts. the floods. but perhaps the mystery of why the lightning disappeared begins with why it happens in the fist place. you think, eric, that it has to do with the darkness of the water and the methane, right? >> translator: black water absorbs more energy. the winds that come down from the mountains and push the energy up into the air, is that the a root of the formation of thth lightning. >eporter: what do you think? >> i think there's intense low pressure in the basin. as night falls, it causes the

towering clouds. >> reporter: separate different lightning storms. >> right now, we have three or four forming around us. >> reporter: truth is, scientists are not sure how any lightning, much less the show here in venezuela is created. it could be solar wind or ice inside clouds. they're hot enough to fuse sand into glass and sing at 145,000 miles per hour. alan is certain it happens allot and just as mysteriously as it stopped, it can start again. as seen f fm this satellite. the intensity of what is to come marked in deep red. for the record, we had alan say

earlier, 12:13, shortly after midnight, it will happen. the cameras ready. trying to see the most predictable show. on cue, shortly after midnight. just as alan predicted. celestial orchestra picks up and jams all night. we realize perhaps this is a mystery not meant to be involved. but enjoyed. i'm matt gutman for "nightline," at the catatotoo river in venezuela. >> wow. coming up, it's the favorite political parlor game, where's sarah? today, she showed up at the iowa state fair. what she told us, next. expensive. be so to save some money, i found one that uses robots instead of real people. 'cuz robots work for free. robot 1:

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at last night's republican debate, there were eight hopefuls on stage and two elephants not in the room. rick perry, who confirmed he will enter the fray. and sarah palin, who hasn't confirmed anything. she happened to show up at the center of the republican universe today. the iowa state fair. what is she up to? jake tapper was there to inquire just that. >> reporter: one of the main reasons for the iowa state fair is the show casing of livestock. since the state of iowa became important for political reasons, a different kind of livestock is now paraded around here.

presidential candidates. all the photographphs. former governor pawlenty is flipping pork. and into this cattle call came a candidate who is not a candidate, at least not yet. former alaska governor, sarah palin. her every move so scrutinize for science as to whether she would run. the media fuels the suggestion hat she's stealing the spotlight from the official candidates. >> he's running to be the mayor of [ bleep ] blockington. why are you here in iowa before the straw poll? >> very thankful we were invited this year. we've been invited the last couple of years. finally, we were able to incorporate the stop into our one nation tour. >> reporter: do you think it is

possible you could come back as a candidate? >> not sure yet. >> reporter: let's get the iowans be the judge. >> love sarah's ideas. don't ne that she should run. >> reporter: why? >> people really like her or hate her. >> reporter: too polarizing? >> yeah. >> reporter: what is your take on her? >> if she's going to get in, i wish she would. >> i will support her if she does get in the race. >> reporter: i don't know why you haven't thrown your hat in the ring? >> i think it's early. totoearly. i thing there's still plenty of time for not just me but others to consider jumping in the ring. >> she's someone that wants to be heard. she believes she has something to add to the debate going on in the country right now. the problem was in this course of this election cycle, the

vacuum is getting filled by conservative candidates that can speak to many of the same voters. >> good to see you. >> reporter: such as michele bachmann,nd texas governor, rick perry, who will declare tomorrow and come to iowa sunday. why would palin not run? >> if she runs and loses, a serious prospect now with the candidates in the race, it diminishing her. >> repororr: skeptics say there are too many questions about her fitness for office. whether she's in this to lead or be famous. nonetheless, she still has something. charisma, leadership, or notoriety. >> i think she's an impactfufu figure within the republican party. in many ways she sets the tone they all follow. when she gets on her twitter

account and tweets, you know, when she gets on the facebook page, people in that party pay attention to her. >> reporter: is it fun watching her for you, because she's kind of disruptive? >> i think she's an interesting political personality. >> reporter: i'll take that as a yes. she said she would make a decision by next month at the latest. >> i think that practically speaking, that has to be a drop dead timeline. in fairsness to supporters on a timeline, this is what i have told todd over and over again. i don't want to be seen as or perceived as stringing people along. >> reporter: a difficult decision lies ahead. for "nightline," jake tapper in iowa. >> but did she try the deep-fried butter? jake, come on. "gma" will have the