The Associated Press footage from the site shows bones and decomposed bodies among scraps of clothing and plastic bags dug out of the ground by a bulldozer after Iraqi troops noticed the strong smell while advancing on Monday into the town of Hamam al-Alil.
The first investigators inside the town, some 30 kilometers (19 miles) from Mosul, say that the mass grave likely holds about 100 bodies, many decapitated.
The site lies behind an earthen embankment near an agricultural college.
Iraq's Joint Military Command spokesman Brig. Gen. Yahya Rasool says investigators from Baghdad would be inspecting the site on Tuesday and in the coming days.