Possibly the most chilling
phrase in the English language.
A mass grave is a place where a lot of people are buried together in one hole, usually to cover up the evidence of their deaths. You'll hear the phrase often in connection with concentration camps, wars, military dictatorships, or any situation where a lot of people are being killed and need to be disposed of quickly.
You also hear the word "exhume" a lot in connection with this phrase. After the war/conflict/regime is over, survivors frequently try to find their missing loved ones this way, with the help of forensic scientists (some very dedicated people who risk life and limb to do this work).