As many as 4,000 “enemies” of the so-called Islamic State were murdered and thrown into a huge sinkhole off the Baghdad-Mosul highway.

Even before it became the scene of the Islamic State’s greatest crime, the Khasfa sinkhole stood out among the crevasses that pockmark the uneven desert plains west of Mosul.

In June 2014, when ISIS took control of Mosul, the second largest city in Iraq, it quickly began to use this dark place for an even darker purpose. The terror group began hunting down policemen and soldiers almost as soon as the city fell, and an orgy of killing ensued as it slaughtered anyone affiliated with government security forces.

Within months of conquering Mosul, the insurgents had turned the sinkhole into an execution site. According to locals, Iraqi police and human rights organizations, ISIS trucked thousands of captured security personnel to Khasfa. Hands bound and blindfolded, the men would be lined up and shot in the back of the head, their lifeless bodies tumbling into the depths of the pit.

Around 4,000 people were killed at Khasfa, according to an estimate by Human Rights Watch, which has been tracking the sinkhole via satellite since it first heard of the massacre.

Khasfa’s body count dwarfs the slaughter at Camp Speicher, the most infamous of previous ISIS massacre. As many as 1,700 Iraqi army recruits were murdered at the former U.S. military base in 2014, the blood of the victims coloring the Tigris red.

Khasfa is the largest of the known mass graves that dot Mosul’s surroundings, but there are several others. A grave near the town of Hamman al Alil, about 30 kilometers from Mosul, is thought to contain the bodies of around 300 local policemen.