Human Rights Watch has estimated that around 4,000 bodies are buried at the site
A sinkhole outside of Mosul in Iraq has reportedly been turned into a mass grave for thousands of victims of Islamic State killers.
Local residents living near the Khasfah - the Arabic word for a sinkhole - have described how ISIS transformed the area into a "place of death" after capturing it in 2014.
Villagers say the sinkhole - once a local curiosity in the area - was used as an execution site and a mass grave where bodies were dumped.
Mohamed Yassin, 56, who lives in nearby Hammam al Alil, said: "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.
"People became afraid of the place; it became a place of death, a place where you'd be executed."
Mr Yassin said he saw executions take place there on at least six occasions, and most of those killed were police officers, soldiers or government workers.
Hussein Khalaf Hilal, 73, said he was taken to the site by ISIS fighters who accused him of breaking their religious rules.
"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 said.
"They took me there because they wanted me to pledge allegiance, to frighten me."
He said ISIS fighters marched people into the pit after forcing them to take pills.
"They would line them up, 10 by 10, 15 by 15," he said.
He was eventually spared and taken to prison instead.
Belkis Wille, senior Iraq researcher for Human Rights Watch (HRW), said she heard about the site being used for mass killings about a year ago.
Prisoners had said their ISIS 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 said ISIS had piled rusted car parts and shipping containers into it, before bulldozing earth on top.
Ms Wille said it was impossible at this stage to know how many bodies were buried there.
"The figure that we hear over and over again in interviews is 4,000," she said.
Iraqi security forces fighting to recapture the western half of Mosul have now retaken control of the area containing the sinkhole.
Their advance is continuing with Iraqi forces saying they have reached the city's southernmost bridge, a key step in efforts to defeat ISIS.