this is "nightline." tonight, the cave. a hidden hospital buried deep underground. one doctor standing her ground. the race to save lives as a civil war destroys syria. hundreds of thousands of people dead, over 6 million people displaced. the warrior doctor and her incredible acts of bravery. defying oppression and risking her life. documented by national geographic, hope shines in t darkest place. "nightline" will be right back.

good evening. thank you for joining us. it's a hidden sanctuary. a hospital deep underground in syria. where one doctor is leading the charge to save lives. tonight, with the help from our partners at nat geo, martha raddatz with the story of this medical warrior.

>> reporter: in the western tip of syria, the rain doesn't come often. instead, airstrikes pound down from the a constant in the civil war that has plagued this nation for eight years. years. years. year. deadly chemicals poison syria's people. the patients covered in dust, suffocating, come here to dr. ammani balure. by some estimates, nearly half a million people have lost their lives, 6.6 million displaced.

above ground, entire neighborhoods left destroyed and desolate. so below ground they went. here, 65 feet below the surface is a hospital they call "the cave." >> it's scary, because you feel, you don't have any, anyplace safe. >> reporter: here in the cave, dr. ammani is in charge, the lead doctor at only 28 years old, a nearly impossible feat for a woman in syria, but not for a woman like her. you wanted to do what girls don't get to do. >> i dreamed of it. i know i can do something different. i have this idea very early when my sister get married 13. >> reporter: 13. >> yeah, i didn't want that. i wanted to come with my study.

and i insist to do that, but all people around me and my family, they refuse that, because because i'm female. >> reporter: against her family's wishes she began studying to be a pediatrician as bashar bashar al assad began waging war against his own people. by then her city was under siege. >> they close everything, every entrance. and it was very bad situation. we don't have anything to eat now. >> reporter: a fellow physician, dr. saleem newmoore had opened a small hospital underground. she went to volunteer. >> it was about 2 or 3-year-olds we have. we couldn't be up, because they targeted everything. >> reporter: hospitals. it didn't matter.

bomb the hospitals. >> they bomb the hospitals lot of times. too many times. the hospital, six years, 20 or times later. >> reporter: a syrian-born film maker had heard about dr. ammani and the plight of hospitals in the region. >> the hospital was established in syria when the syrian regime start to use the war plane, targeting intentionally the hospitals. because the hospital where the victims come, where the people gather, where the people are protected. >> reporter: he began filming "the last men in aleppo". a film that focussed on the rescue group "the white helmets." he says he witnessed the regime's brutality first hand. >> i was jailed, torture, they take my nails.

they use a brutal way of torture. i don't know how i survive. >> reporter: but he couldn't forget dr. ammani and decided that his next film would focus on the network of underground hospitals, and this indomitibling woman. >> i see something powerful in the gender and the situation of heroine female. >> reporter: his team's footage would be used for the next film "the cave", done in conjunction with national geographic which flew us out to meet dr. ammani. it details the doctor's time underground, their hardships and

successes. dr. ammani is elected by her peers to lead the group, but there are patients who don't feel she is worthy of the title. >> they make me frustrated, because i can. i can do something. i wanted to do something. and the doctors in the hospital, they choose me to be the manager of the hospital. >> reporter: it must be so important for little girls in syria to see a woman do what you do. >> yeah. i wanted that. i wanted them to see that. we have to break the barriers. we have to do something different. i wanted to support them and say you can, you can do that.

>> lot of little girls say to me, we want to be manager. we want to be doctors. >> reporter: for dr. ammani, their this is at the heart of what she and so many syrians wanted in 2011 when the regime zifirst >> i was very happy, i thought that out of revolution will be the same. we want freedom. it's very simple thing. we want dignity. when someone prevents you, he

y you feel angry at that. something inside you push you. >> reporter: this was the doctor's rebellion. fighting to save lives, even as the regime seemed determined to take them. there's a moment in there after all you had seen when a woman loses her son. when you break down. what was it about that day and that moment and that child? >> yeah, i was, you saw me crying in this moment, because i cry a lot before, you know, we are human.

not machine. i am doctor, i have to do my job. i'm not a martyr, but i know that what she felt. >> reporter: how many children do you think you lost that you saw? during that period? >> oh, lot of children. but i saw thousand of children. and i call them my children, because i love them. and unfortunately, i still, some of them dead in 2013 in the chemical attack. and we don't know what the problem is. we try to treat the symptoms.

but we don't know that. it was sarin. >> reporter: this was after the assad regime had supposedly destroyed their high-risk chemical weapons. after this attack, doctor ammani made the gut-wrenching decision to leave. >> i love the hospital and everything in it. every place, i loved it. i wanted to stay, and i tried. me and my staff and all of us. we started really very brutal attack on us, on civilians especially. you cannot see anyone above the ground. when i get out the hospital,

it's very hard, it's very hard. >> reporter: next, dr. ammani's journey out of syria, and how her life has changed now she is a refugee. the roomba i7+ with cleanng base automatic dirt disposal and allergenlock™ bags that trap 99% of allergens,

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>> reporter: as another day passes, dr. ammani faces her new life. >> i decided, so my husband, we decided to get married. we did it. and we are good now. >> reporter: she is a refugee now, living in a far cry from the life she lived as a doctor in a subterranean hospital in her home nation. the war-torn syria.

she managed to escape to nearby idlib and eventually was granted refugee status in turkey, but not everyone was so lucky. what happened to some of the staff who stayed? >> some of them were arrested. one, i know two doctors, one of them now in prison, and other one, they arrested him and when we went. and they killed him in the prison. >> reporter: dr. ammani, you have seen so much. and you're out, now you're safe. how do you deal with those memories? >> yeah. i have lot of memories. most of them are bad memories. i dream a lot of these things sometimes. i woke up at midnight scaring, because i hear bombing or something like that.

and it's very bad situation now in syria. too many countries, they want benefits from it. >> reporter: president trump made headlines when he announced the troops were pulling out of syria. trump would later send several hundred troops back to protect oil reserves. >> keep the oil. >> reporter: many experts view the syrian conflict as a proxy war of sorts, with several nations' interests at play. russia backing syria, the u.s. backing the rebels, while iran and turkey, looking to stake their own ground. turkey currently hosts the largest number of syrian refugees, 3.6 million. human rights groups tell us that the turkish government has been quietly planning to send refugees back to syria in

designated safe zones. refugees that live here are already restricted to certain areas and are required to carry special identification. the refugee crisis is most evident in these border towns. they used to be all turkish. now many of them are majority syrian. while the crisis has interrupted education for millions of children here of, just miles from the syrian border, we met a group whose love of learning is enduring. they used to own a business in houston but decided to return to his native syria to start the merim foundation which sponsors this school. >> we do a holistic approach, meaning we give food, and we give fun, we dosuppor support.

they're learning math and are happy. that doesn't happen everywhere. >> reporter: dr. ammani saw little happiness during her time in syria. the memories still haunt. >> it's very difficult to work with children. every child i saw, i remembered someone, i remembered how they were suffering. i couldn't start again. it's difficult to see children again. >> reporter: i have to ask you, dr. ammani, do you want to have children? >> of course. >> reporter: despite everything. despite what you've seen. >> i wanted that of about. you know children in syria will die because of bombing, because of starving, because of disease, because of no medicine. i said no, i will not have children all of my life. but yeah, now i want a child. this is a dream to me.

i hope i can get a girl and support her to be strong. remember her dream is for a syria where that little girl and all children cannot just exist but live in a free country that's all their own. >> reporter: and what will you tell your girl about your country? >> i will tell my children in the future and all the children i saw about the truth in syria and about my experience and my medical staff what we work, what we do. they have to know. >> reporter: and you'll show them that film? >> of course, it's very important to do something like that, to do films, because it will stay, the assad regime, and

they will go. but the truth will stay. of >> our thanks to martha and our partners in national geographic. a final note when we come back. f >> our thanks to martha and our partners in national geographic. a final note when we come back. >> our thanks to martha and our partners in national geographic. a final note when we come back. partners in national geographic. a final note when we come back. may your holidays glow bright and all your dreams take flight. lease the c 300 sedan for just $399 a month at the mercedes-benz winter event. hurry in today. he borrowed billions donald trump failed as a businessman. and left a trail of bankruptcy and broken promises. he hasn't changed. i started a tiny investment business, and over 27 years, grew it successfully to 36 billion dollars. i'm tom steyer and i approve this message.

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in the eight years of civil war in syria, the numbers are staggering. by some estimates, half a million people dead, more than 6 million driven from their homes. and then you have the examples like aymani balure, trying to make a difference one life at a time. the complete story of "the cave" is in select theaters now.