The Jews usually burned the crosses used by the Romans after executions but following Jesus' crucifixion they quickly threw the Cross in a ditch to get it out of sight before the feast of Passover. That preserved the True Cross, and memory of the events preserved its location.
Disgusted with continued Christian veneration of the spot, pagan Roman Emperor Hadrian erected on the Cross's burial site a statue to Venus, hoping thereby to obliterate their memory. It didn't work.
Indeed, because of the statue, when the Empire became Christian, St. Helena knew the exact spot where she would find the very Cross on which Christ died. All relics from Christ's crucifixion have a similarly fascinating story, all of which are told here in this 1910 work by the enterprising Catholic investigator Charles Wall.