It's hard to understand how a cavern used as a temple and filled with rich artefacts could be covered over and lost.
When and how come to mind? Could it have been closed off to prevent barbarian invaders from finding it?
It was probably sealed off on orders of the same Pope who abolished the pagan rituals conducted there.
The ceremony survived until A.D. 494, when Pope Gelasius put an end to the tradition.
I suspect the answer to your question's in there somewhere.