Ruins of the Temple underneath the Temple Mount?

Here's a little archaeology/religious history question.. Why would the ruins of any Temple be underneath the Temple Mount? Now I understand with time, soil and debris cover "some" places, but thats only in terrains where earth is naturally building up. They found that Byzantine street in Jerusalem underground but the Temple Mount is a mountain, it must have always been a hill. Wouldnt the ruins of any temple been cleared for a church, then mosque? look at the western wall! thats not covered by soil. And if that was truly the wall then the temple would have been above it.