In the civilized world, hospitals are off-limits in war. But in Syria, the dictatorship of Bashar al Assad is hitting hospitals, relentlessly, killing more than 800 doctors and medical workers.

On assignment for "60 Minutes," Scott Pelley and his team traveled into Syria with American doctors who volunteer to risk death to save lives.

Hospitals in rebel-held territory have been driven underground. They're makeshift affairs with equipment and drugs salvaged from medical centers that were leveled in cities including Aleppo.

"You work with the understanding that you might find yourself dead, or… or crippled, or dismembered on the floor next to the people you're trying to save. Because the bombs would land so close they'd… they'd knock you off your feet," said Dr. Samer Attar.

Attar was born in Chicago to Syrian parents. He's a leading orthopedic surgeon and a member of the Syrian American Medical Society. The society used to just connect Syrian-American doctors to one another but it rose into action with the war, and today, it supplies doctors and millions of dollars in aid to the suffering.

Dr. Samer Attar 60 Minutes

"I've learned that Syrian children... the Syrian people are, are very resilient. They're able to find humor and strength in even the darkest of circumstances," Attar said.

The Syrian American Medical Society is now building replacement hospitals inside Syria where you might not expect, like this cave, where the limestone had eroded away over thousands of years.

Hospitals in rebel-held territory have been driven underground 60 Minutes

Then, engineers came in, cleared out the cave and lowered the floor about six feet. When the hospital is finished, it will have three operating rooms, 12 inpatient beds and a state-of-the-art emergency room. They expect it'll be hard to find and even harder to hit.

Since 2011, the Syrian American Medical Society has sent more than 100 doctors into the battlefield and raised more than $97 million in aid.