Mass Grave

The Mass Grave is commonly misinterpreted as a myth. It is located near the Restricted Area and Verdant Meadows Aircraft Graveyard. According to some sources, the bodies were dumped there by a serial killer. Inside the grave, which is a large round hole in the ground, are six body bags, next to the grave is a parked truck (a Bobcat), the same truck can be found parked next to a small shack where the mythical serial killer is supposed to inhabit.

Answer
Although many people choose to link this myth to the 'serial killer' myth, this is not the case, as two Sindacco Family thugs are seen digging the grave, and then leaving it in The Introduction that came with the Grand Theft Auto: San Andreas Soundtrack, although the two goons are only burying one person, not six.