A retired soldier, he said he saw people being executed at the site on several occasions after IS captured the area in June 2014.

"They would bring them blindfolded, their hands tied behind their backs. The Khasfah would be in front of them, they would make them kneel down, shoot them in the head and push them in," said Mohamed Yassin, 56, a resident of the nearby town of Hammam al-Alil.

But the Islamic State group transformed it into a "place of death" after capturing the area in June 2014, using it as an execution site and a mass grave where they disposed of victims, according to local residents.

The place known as the Khasfah (an Arabic word for a crack or a hole that opens up in the ground) was once a local curiosity, a natural formation that many locals believe was caused by a meteorite.

The sinkhole that could be the largest mass grave in Iraq's latest conflict is barely visible from the road, nothing more than a small depression behind a desert ridge near Mosul.

The Khasfah sinkhole in Athbah could be the largest mass grave in Iraq's latest conflict (photo by: ARIS MESSINIS/AFP)

He was in the area regularly, transporting oil from a site just metres away, and said he saw executions there at least six times.

Most of those killed, he said, were policeman, soldiers or government employees, judged guilty for their association with the Iraqi state.

"People became afraid of the place, it became a place of death, a place where you'd be executed."

Sinkhole bulldozed over

Hussein Khalaf Hilal, 73, was taken to the Khasfah by IS fighters who accused him of violating their rules by treating people with religious folk medicine.

"They came to the house, they blindfolded me, tied my hands behind my back and took me away in a car with blacked out windows," he told AFP.

"They took me there because they wanted me to pledge allegiance, to frighten me."

Hussein Khalaf Hilal was taken to the Khasfah by IS militants but eventually put in prison (photo by: ARIS MESSINIS/AFP)

He said IS fighters marched people into the pit after forcing them to take pills.

"They would line them up, ten by ten, 15 by 15," he said.

He declined to pledge allegiance, but asked for a chance to consider the matter, and was taken to prison instead.

The stories of mass executions match what Belkis Wille, senior Iraq researcher for Human Rights Watch, has heard for months.

"I started hearing about this location about a year ago, in interviews I was doing with people who had fled IS control," she said.

They told her about people who had been executed at the sinkhole, and prisoners whose IS guards told them they were taking detainees to the Khasfah to be killed.

HRW examined satellite imagery that suggested the sinkhole was filling up, and local residents told AFP that IS had piled rusted car parts and shipping containers into it, before bulldozing earth on top.

The battle for Mosul (photo by: Paz PIZARRO, Sophie RAMIS, Thomas SAINT-CRICQ/AFP)

A month after the area was taken from IS, the once-cavernous hole now extends just a few metres down for most of its surface.

In the centre, there is a smaller, deeper hole, with the carcass of a vehicle lying on top.

The area is strewn with IEDs, both inside and around its perimeter, and is in territory patrolled by Iraq's Hashed al-Shaabi paramilitaries.

'Place of sadness'

"This is a place where you feel sadness," said Hashed fighter Abu Ahmed Hassani.

"You think about all the Iraqis who have been executed, from all the sects," he said.

"They killed children, they killed old people, women, men."

And the Khasfah is still claiming lives.

On Saturday, a reporter from Kurdish channel Rudaw and three Hashed members were killed at the mouth of its central hole, when an IED detonated.

Smoke billowin over Mosul while Iraqi forces fight jihadists as they advance in the city on February 26, 2017 (photo by: ARIS MESSINIS/AFP)

No exact figures yet exist for the number of bodies that could be buried in the sinkhole.

"The figure that we hear over and over again in interviews is 4,000," said Wille, stressing the information was as yet impossible to verify.

HRW wants to see Iraq's government, which has an inter-ministerial team dedicated to dealing with mass graves, carry out an extensive operation to protect and excavate the site.

"We'd want to see that team going up as quickly as possible, marking off the site both to protect it and also to stop people going to a site that's contaminated," said Wille.

"After that comes the harder job, which is first collecting the surface remains... for use by forensic experts to start identification, and after that the much harder work of excavating the remains that are below."

But Hassani said he thought it would be impossible to excavate the many layers of the sinkhole.

"What should happen is that it should be covered o

ver, and become a cemetery for Iraqi martyrs," he said.