What are the "killing fields" of cambodia? answer throughly , not vague!

It is a number of sights in Cambodia where collectively more than 1 MILLION people were killed and buried by the Khmer Rouge Regime ( which was the government faction that was ruling between 1975-1979). This was commonly known as "State Sponsored Genocide". 20,000 mass gravesites indicate almost a million and a half executed victims, but think it could've been 2+ million out of 8 million.