Thousands are believed to have been buried in the sinkhole, with some executed at the edge and others tossed in alive.
Witnesses say some of the victims were executed on the side of the pit and pushed inside, while others were thrown in alive.
Underground water flows are thought to have washed many of the bodies away, and parts of the sinkhole filled in.
"They would bring them and make them get out and line up at the edge of the hole ...and then they would execute them," resident Mohammed Younis said.
"I remember one incident when they brought some women. They brought them in a minibus.
"They took them out of the minibus, three women, two young ladies and an elderly woman, and there was an old man with them.
"I went to them and asked, 'Who were they?' And they said, 'They were informing on us to the security forces.'"
Mr Younis said the militants went on to fill the hole with cars and sand. It is one of at least 72 mass graves created by Islamic State in Iraq and Syria.
Mosul, Iraq's second largest city, was liberated in July after a nine-month battle. The city had been in the hands of Islamic State militants since 2014.
A UN human rights report said IS carried out mass abductions and killings, and used civilians as human shields.
Iraqis have been trying to access the sinkhole for months, but have been prevented from reaching it by landmines that scatter the land surrounding it.
In March Fawaz Abdelabbas, the deputy head of the International Commission for Missing Persons in Iraq, told Sky News that he hoped to identify bodies of the people who had been thrown into the hole.
"Returning the remains to grieving families will help them deal with the past but it will also demonstrate that the (Iraqi) government actually cares and respects them," he said.