Soon after the group took control of the Iraqi city more than 2 1/2 years ago, the 100-foot-wide sinkhole five miles southwest of the airport became a site for executions. Some victims were made to line up at the edge of the hole and were shot before being kicked inside, while others were tossed in alive, residents said. Sometimes bodies were just trucked in for dumping.
Residents of Mosul whispered about the deaths at the sinkhole, or "khasfa" as it is called. But with communications limited and locals too fearful to speak out publicly, it was only after Iraqi forces retook the area last month as they closed in on the city that the scale of the killing at the site began to emerge. Based on anecdotal evidence, Iraqi officials say thousands may have perished there in recent years.
It may be years more, though, before the mass grave gives up its secrets.
No one knows the depth of the hole under the water at the bottom. The militants have now filled it over and booby-trapped it with explosives, making excavation particularly complex.
Even before the Islamic State's brutal campaign began, Iraqi authorities were struggling to excavate and identify victims of mass graves dating back to the reign of Saddam Hussein, when as many as 1 million Iraqis went missing. Sectarian war following the 2003 U.S.-led invasion brought more large-scale bloodletting.
Dozens of mass graves around the town of Sinjar, which are thought to contain the remains of hundreds of Yazidis executed by the Islamic State, are yet to be fully excavated. Mass graves around the Iraqi city of Tikrit containing the remains of an estimated 1,700 soldiers from nearby Camp Speicher who were massacred by the militants, are still being discovered two years after the area was retaken by security forces.
The khasfa, though, could be the group's biggest mass grave.
"It's swallowed the lives of thousands," said Muthanna Ahmed, who said he worked near the site for five months and witnessed executions. "It was terrifying, very deep and dark."
Ahmed said victims' shoes and dried blood lined its rim, while some decaying bodies that got caught on the sinkhole's rugged edge were still visible. A gruesome video posted on YouTube in January 2015 shows a similar scene.
The sinkhole was near an Islamic State oil refinery, and the militants regularly rounded up workers and Mosul residents who were buying fuel to watch the executions. Victims included former police and army officers, as well as those accused of spying or working with the Iraqi government, witnesses said.
Hussam al-Abar, a provincial council member, said that between 3,000 and 5,000 corpses might languish in its depths, though he bases that number on lists of missing people that he concedes could have been killed and buried somewhere else.
"Given the capacity of the central government and local government, I think it's impossible to take out the bodies," he said. "We'd need international assistance. It would be impossible for Iraqis alone."
Before 2003, the sinkhole was a small tourist attraction, drawing travelers from the main Mosul-Baghdad highway a mile and a half away, Abar said. But as violence gripped Iraq in the wake of the invasion, al-Qaida began to gain a foothold and the site became a desert grave.
"It was known that whoever wanted to hide a body could drop it in this hole," Abar said.
Jassim Omar, 33, said he witnessed about 10 executions there. The first was about a month after the city fell to the militants. About 25 prisoners from Badush prison in Mosul were brought to the sinkhole and killed, he said.
"If you want to scare someone from Mosul, just mention the khasfa," he said.
The militants killed hundreds of the prison's inmates when they took over the city, according to human rights groups. Most of the victims were Shiites, Yazidis and Christians, all of whom the militants consider to be apostates, while many Sunni inmates were allowed to go free.
In an execution in March or April of 2015, Omar recognized his cousin among a dozen detainees brought to the site in the back of a truck, blindfolded and bound. His cousin had worked for Mosul's local council before the militants took over and was accused of collaborating with the government.
"Whenever we went, we expected to see executions," he said. "But we were surprised to see our cousin. We just thought he'd been arrested."
He said he watched as the men were made to kneel and three or four militants shot them, while a few others pushed the bodies into the hole.
The stench could be smelled several miles away, he said.
The smell might have been what led the Islamic State to fill the hole in mid-2015. Residents of Athbah, the nearest village, had complained, and some had even left, said Jawad al-Shammari, a spokesman for the human rights committee, which has not yet sent a team to examine the site.
Residents said the Islamic State pushed dozens of trailers and old cars into the hole before filling it with earth, though some said mass killings at the site continued until as recently as six months ago.
Human Rights Watch, which began monitoring the site by satellite in September 2014, said the hole had been filled over by July 2015, though fresh track marks appeared there until November 2016.
"It will take ages for them to decontaminate the site and excavate," said Belkis Wille, senior Iraq researcher for Human Rights Watch. "De-miners are rightly prioritizing decontaminating areas that displaced people are returning to."
With explosives planted in the area around it, the khasfa claimed its latest victims last week. Shifa Gardi, a 30-year-old reporter for the Kurdish television channel Rudaw died with a militia commander and four other soldiers when the group set off a booby trap near the site.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.