Hoping to unearth crucial evidence that could help in convicting deposed Iraqi leader Saddam Hussein, investigators said nine trenches in a dry riverbed at the Hatra site in northern Iraq contained at least 300 bodies, and possibly thousands.
Those buried included children still clutching toys.
"It is my personal opinion that this is a killing field," said Greg Kehoe, a US lawyer appointed by the White House to work with the Iraqi Special Tribunal.
"Someone used this field on significant occasions over time to take bodies up there and to take people up there and execute them".
"I have been doing grave sites for a long time, but I have never seen anything like this, women and children executed for no apparent reason," Kehoe said. "It's a perfect place for execution".
The victims are believed to be minority Kurds killed during 1987-88. One trench contains only women and children, apparently killed by small arms. Another contains only men, apparently killed by automatic gunfire.
Some of the mothers died still holding their children. One young boy still held a ball in his tiny arms.
International organisations estimate more than 300,000 people died under Saddam's 24-year rule and Iraq's Human Rights ministry has identified 40 possible mass graves countrywide.
Authorities hope careful investigations of the site will provide enough evidence to convict Saddam and other senior members of his regime.
Though investigators have excavated smaller mass graves before, scientific exhumation is being done for the first time.
Saddam (R) is likely to face trial
for genocide next year
Saddam is expected to face trial for crimes against humanity next year. Investigators are still pursuing evidence.
He is accused of widespread abuses against the Kurds, including a particularly gruesome campaign in 1988 when mustard gas was deployed against the population.
During his reign, Saddam pushed hundreds of thousands of Arabs into Kurdish areas to force the locals out. He is accused of widespread abuses against the Kurds, including the "Anfal" (The Spoils) campaign in 1988, during which thousands died in a mustard gas attack.
Human Rights Watch estimates that more than 50,000 Kurds were killed during the campaign.
Iraqi sources revealed today that the former Iraqi ruler underwent an operation to repair a hernia about 10 days ago but has made a full recovery, Iraqi sources said Tuesday.
The ousted dictator was taken to the Ibn Sina hospital near the US-controlled Green Zone for the procedure, which was performed by Iraqi doctors, according to sources close to the Human Rights Ministry.
The operation lasted about an hour and Saddam was returned to his cell the same day, the sources said on condition of anonymity.