[Cthulhu_]

An interesting theory (albeit likely sci-fi) is that on a long enough timescale, any existence of an advanced precursor society will have been lost by tectonic plates sliding down. But that's a timescale of hundreds of millions of years (apparently the earth is ~4.5 billion years old, human life that left traces of intelligence behind is a percentage of a percentage of that)

[stormfather]

Its the ocean plates that slide down when they hit the more buoyant continents. When continents hit they make the Himalayas.