Some scientists suspect the tectonic plate system is an anomaly left behind by a terrible catastrophic event.
This is discussed by Robert Felix in his books - the suggestion there is that the crust itself may have occurred as a negative charge on the outer surface when the inner core of the Earth was charged positive by a sudden magnetic reversal. The two of them were antagonistic and the outer skin of the planet was permanently separated by the force of this repulsion, left to float on a bed of magma as a geological formation in it's own right.
If this were the case it would go a long ways in explaining how the tectonic "conveyor" belt works, dragging the surface of continents into the sea and under the plates in a grand cyclic action that never ends, continuing to respond to the differently charged interior and exterior. We'd also know for certain that most of the evidence of prior civilizations is destroyed in this fashion.
When we find ancient monolithic structures on the floor of the sea, what we are really doing is discovering the ruins of the prior world as it marches towards the rifts at the bottom of the ocean to be subsumed. It is like attempting to run halfway down a conveyor belt to look at rubbish on it's way to the incinerator.