In the shadow of an earthen dam and buried under rubble and trash scattered across the al-Sadah waste ground lies one of the most frightening places in Baghdad at the peak of Iraq's sectarian slaughter.

Beneath the detritus and shacks since constructed on the killing field is buried what may be one of the largest unopened mass graves in the Iraq capital, a macabre testimony to the darker days of the country's war.

Tens of thousands of Iraqis were killed or went missing in the sectarian conflict in 2006-2007 unleashed by the U.S.-led invasion that toppled dictator Saddam Hussein in 2003.

Many of the missing were never found, and as the last U.S. troops leave Iraq, the excavation of mass graves that may provide answers for the relatives of the dead is considered a critical step in healing after years of war.

Some believe al-Sadah in eastern Baghdad, one of 41 unexcavated mass graves known to the Iraqi Human Rights Ministry in the capital and its outskirts, contains hundreds of bodies buried just a few centimetres beneath the dirt.

You cannot say there are thousands or hundreds of bodies, as people think, said Mohammed, a former Shi'ite militia fighter, fidgeting and averting his eyes from the site piled with trash and reeking of burning plastic.

Police took away most of the victims. But still, there are many.

Mohammed, who did not want his last name used for fear of reprisal, said he witnessed many killings at the sprawling site.

With the end of what many see as a U.S. occupation, Iraqis are trying to dig up dozens of graves in a concerted effort to learn the fate of some 9,000 missing people -- an effort they believe will help heal the Sunni-Shi'ite divide.

NUMBERS IN A FILE

Some of the missing simply disappeared but thousands of others ended up in numbered mass graves for unknowns, their identities reduced to a file at the morgue, waiting for the day their families go looking for them.

The Human Rights Ministry and security officials say al-Sadah is not the largest mass grave left by militant groups since 2003, but it might the biggest created by the Shi'ite militias in Baghdad and still untouched.

The dam extends for a distance of 25 km (15 miles), bordering many large Shi'ite neighbourhoods, and was used by all the Shi'ite militias without exception, Mohammed said.

Police officials who worked there and witnesses who live nearby said the militants used to bring victims from across Baghdad by car to be investigated nearby, and then they were sent to al-Sadah and killed with a single bullet to the head.

Al-Sadah is a part of Sadr City, a sprawling Shi'ite slum east of the Tigris River that used to be the stronghold of anti-U.S. cleric Moqtada al-Sadr's Mehdi Army militia.

At the peak of the sectarian conflict, the Mehdi Army was seen by Washington as one of the biggest threats to security, with its young fighters toting rocket launchers and battling U.S. and Iraqi troops in the streets.

Many Iraqis believe those buried in al-Sadah were only Sunnis. But ex-Mehdi Army fighters, security officials and witnesses say the site was simply a crime scene.

They were Sunnis, Shi'ites and Christians, senior al Qaeda leaders, Shi'ite informers, security officials, ex-Baathists, translators, pimps, prostitutes, gays, drug dealers, rapists and even Mehdi Army fighters, said Abu Zahraa, a former senior Mehdi Army fighter.

BIG GRAVES, SMALL GRAVES

While American troops pull out and U.S. officials declare the war over, bombings, assaults and assassinations by Sunni insurgents and Shi'ite militias still occur nearly daily.

Mass graves have been unearthed regularly since 2003, often containing the bodies of large groups of people kidnapped and killed by al Qaeda and other groups, or the victims of mass executions carried out under Saddam Hussein.

Insurgents used fields, schools, deserted houses, garbage dumps, streams and rivers as burial grounds.

Earlier this month a farmer named Bissam was watering his palm grove on the northern outskirts of Baquba when he noticed the ground was uneven. A strong smell of decaying flesh emanated from areas where the earth absorbed the water.

Bissam returned a year ago to the area, which had been a stronghold for Sunni Islamist al Qaeda.

It didn't seem normal and it raised my curiosity and prompted me to dig in the spot. When my spade dug just a few centimetres ... I was surprised to find a corpse, he said. I put everything back in its place and called the police.

When they arrived, police unearthed nine badly decayed bodies. They had been bound and blindfolded, and forensics experts found bullet wounds in their heads and chests.

We do not rule out that there will be a mass grave in any place that has been under the control of terrorism, said Kamil Ameen, spokesman for the Human Rights Ministry.

Restive Diyala province east of Baghdad, and the areas of Latifiya and Iskandariya, south of the capital, are likely sites for more mass graves.

The worst thing that terrorist actions caused was the great schism in the Iraqi society, Ameen said. Anything that reveals the fate of the victims will contribute significantly in healing the relatives' wounds.

(Editing by Jim Loney and Sonya Hepinstall)