The Mass Grave is a pit located at the foot of a mesa in El Castillo del Diablo, Bone County, north of the Restricted Area and south of the Verdant Meadows aircraft graveyard in Grand Theft Auto: San Andreas. The pit, which takes the form of a large round hole, contains six body bags, and a Bobcat truck and shovel pickup next to the pit.
The grave could be a place for the Crime Familes to dispose of bodies, as two Sindacco Family thugs (depicted in "The Introduction") are seen in the desert, digging a grave to dispose of a body, although it's one person, not six - possibly indicating that more people were buried there during the events of GTA San Andreas.
The possible person who could have made a graveyard here could be Mary-Beth Maybell, from the radio station K-Rose. She says that she has six husbands, all of whom she has killed and says that they are now "six feet under". There are six body bags in the grave. She also says that she broadcasts somewhere in Bone County, which is a desert, and the same location of the mass grave. She also says she drives a truck, which may explain the nearby parked Bobcat.
Mass Grave article on GTA Myths wikia.