NEAR MOSUL, Iraq — 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.

Villagers from the surrounding hamlets could not see the bottom of the pit when they peered cautiously into the abyss. In 2003, a farmer's boy fell into sinkhole, which lies next to a road cutting through the parched landscape, a mile off the Baghdad-Mosul highway.

When the rescue services tried to retrieve the child’s body, a rope 450 meters long was not enough to reach the bottom, local legend goes.

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.

“I could see that the edge of the hole was drenched in blood,” says Hamad, a 29- year-old from the nearby village of Sananik who worked at a makeshift refinery just down the road from the crime scene.

The jihadist were keen to let the locals know what was going on at Khasfa, which lies roughly five miles from the edge of Mosul.

“We used to buy petrol at the refinery, and Daesh [ISIS] would stop us and make us watch the executions,” says Mahmoud, who also lives in Sananik. Like Hamad, he is still too terrified of ISIS to reveal his full name.

The terror group also posted lists of the people it had executed at the mosques in and around Mosul, says Mahmoud.

The 40-year-old was pulled off the road to witness the slaughter four times. He was forced to watch the murder of his cousin, a police officer.

By the end, the bodies had piled so high that they could be seen from the edge of the hole, he says. The once seemingly bottomless pit had been filled with carnage.

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.

As ISIS sought to bring a city of 1.5 million inhabitants to heel, it wasted no time in exterminating Mosul’s police force. But, motivated by hatred towards anyone who does not subscribe to their hardline interpretation of Islam, the jihadists also delivered other victims to the slaughter at Khasra in convoys of minibuses, trucks and pick-ups. Mahmoud witnessed how the jihadists drove a bus full of Yazidis up to the hole, and then rolled it over the edge. Tied up and trapped in the vehicle, the men inside fell to their death. The killing was so indiscriminate that even parents of police officers were butchered at the sinkhole.

“I even knew a man who was killed because his son was with the police,” said Mahmoud.

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.

When security forces pushed towards Mosul on the west bank of the Tigris, they quickly realized that the rumors that had emanated out of the ISIS-held city about the sinkhole were true.

“The civilians in the villages told us that Daesh threw the bodies of dead policemen into that hole,” says Capt. Mahmoud Sadi, whose unit of federal police, a heavily armed paramilitary outfit, helps secure the area around Khasfa. A current estimate puts the number of colleagues lying dead in the pit at over 2,000, says Col. Uday Saber, a police officer who escaped from Mosul in 2014. But that estimate does not include army personnel or other victims.

When the hole had been filled with death, the killers dumped several dozen shipping containers and sections of concrete blast walls into the pit to weigh down the bodies. Bulldozers then shovelled earth onto the pit. By June 2015, the crevasse had been turned into an inconspicuous depression in the desert roughly a hundred meters wide. In its middle, a smaller pit appeared, as the downward pull of the sinkhole is at work again. Tire marks line the rim of the crater, and lead to the smaller hole, where two car wrecks lie shattered.

Mystery surrounds what HRW says were thousands of shipping containers which appeared on satellite imagery of the site between September 2014 and March the following year. Some were positioned around the hole, others alongside the road, according to Belkis Wille, Senior Iraq Researcher at the organization. The containers not dumped into the pit have vanished.

After ISIS levelled the sinkhole, the terror group continued the killing elsewhere. A supermarket in a Mosul neighborhood became one prominent execution site, according to Mahmoud. Victims would be thrown from the seven-story-high building, or made to walk off the roof blindfolded, unaware that they were stepping towards death.

The insurgents lost control of the area around Khasfa when the Iraqi military, backed by the airpower and special forces of the U.S.-led coalition, on Feb. 19 launched its offensive to vanquish the terror group in west Mosul, its last major stronghold in Iraq.

The signs of battle are easy to find. Hundreds of spent heavy machine gun cartridges lie on the road leading to the sinkhole. Iraqi special forces trained their guns at a mountain range in the distance to practice their marksmanship a day before they pushed into Mosul.

The executioners have left, and the fighting has shifted into the city. But danger still lingers at Khasfa. On Feb. 25, Kurdish journalist Shifa Gardi was killed at the sinkhole when explosives that had been planted by the insurgents detonated. Not content with the slaughter of thousands, ISIS seeks to extend the death toll to those investigating its crimes.