BAGHDAD — Over 200 mass graves holding as many as 12,000 bodies have been found in areas of Iraq once controlled by the Islamic State, the United Nations said on Tuesday. The findings were highlighted in a joint report released by the United Nations mission to Iraq and the Office of the High Commissioner for Human Rights, which called the sites a “legacy of terror.”

Where are the graves, and what are they like?

Most are in the four provinces of northern and western Iraq where the Islamic State’s so-called caliphate acted as the government: Anbar, Kirkuk, Salahuddin and Nineveh, which includes Mosul, the largest city once controlled by the extremists. They range from small burial sites with eight bodies, to massive pits believed to hold thousands. The biggest is believed to be the Khasfa Sinkhole near Mosul.

“I can only say that the number of the victims of the mass graves is much bigger than the numbers in the report,” said Dhia Kareem, head of the Mass Graves Directorate in Iraq. He said eyewitnesses estimated there were 6,000 bodies in the Khasfa Sinkhole.

“ISIL’s horrific crimes in Iraq have left the headlines, but the trauma of the victims’ families endures, with thousands of women, men and children still unaccounted for,” Michelle Bachelet, the United Nations human rights commissioner, said, referring to the Islamic State. “These graves contain the remains of those mercilessly killed for not conforming to ISIL’s twisted ideology and rule.”