ON FEBRUARY 26, ISIS released a video of its militants smashing ancient Assyrian artifacts in the central museum in Mosul, Iraq. In a matter of minutes, they jackhammered the face of a famous 1,400-year-old Assyrian winged bull and broke apart four 2,000-year-old statues of the kings of Hatra. That same week, insurgents from the so-called Islamic State burned thousands of rare books and manuscripts from Mosul's library.

A week later, Ahmed Salem, a 28-year-old former archaeology graduate student, crossed into Syria and entered ISIS-held territory armed with nothing more than a notepad, a camera, and a phone, the contents of which, if he were discovered, could get him killed. His task: to photograph evidence of cultural heritage crimes in his home country.

Salem, whose name has been changed to protect his identity, is part of an underground network of activists secretly documenting the ransacking of Syria's ancient treasures. ISIS's handiwork appears often in their photos. What antiquities the militant group doesn't destroy, it steals to peddle on the international black market—a trade some experts claim is increasingly vital to the organization's finances. "The looting is much more intensive and criminalized in ISIS-held territories," says Amr Al-Azm, an archaeologist who has been working with the network from his home in the United States and in Turkey. And these ISIS-held areas are all the more risky for the underground preservationists like Salem. "We have to be really careful, especially with ISIS," Al-Azm explains. "It's a lot of money and a real criminal underworld, not a bunch of geeks playing game cards and getting a little frisky with them."