Here's why:

* Deep rocks are generally very stable: Rocks far underground are under immense pressure from the weight of the rocks above them. This pressure makes them very rigid and resistant to movement.

* Plate tectonics and geological processes: However, the Earth's crust is divided into tectonic plates that are constantly moving. This movement can cause rocks to shift and deform over very long periods, especially along plate boundaries.

* Earthquakes and volcanic eruptions: These dramatic events can also cause significant movement of rocks deep underground, creating faults and fractures.

Therefore, while rocks deep underground are relatively stable, they are not completely stationary. They can move slowly over long periods due to tectonic activity or rapidly during seismic events.