The Khasfa sinkhole was once an inconspicuous feature in the barren desert just off the Baghdad-Mosul highway.

Now, this natural depression five miles outside Mosul is believed to be the biggest mass grave in Iraq and the resting place of an estimated 4,000 bodies.

Islamic State of Iraq and the Levant (Isil) killed and dumped the bodies of thousands of security personnel here after they captured the city in 2014, according to local villagers, Iraqi police and human rights organisations.

Most victims were shot and dumped into the pit, witnesses said, while others perished in vehicles driven over the edge.

"Daesh would drive the victims to Khasfa in convoys of minibuses, trucks and pick ups. The men had their hands bound and their eyes blindfolded.