Here's why:
* The Earth's crust is relatively thin. The average thickness of the Earth's crust is only about 18 miles (30 kilometers) under continents and about 4 miles (6 kilometers) under oceans.
* Pressure and Temperature Increase with Depth. As you go deeper into the Earth, the pressure and temperature increase dramatically. At depths greater than a few miles, the rock is no longer brittle enough to form caves. Instead, it becomes more plastic and flows.
* The deepest known caves are relatively shallow. The deepest known cave in the world is the Veryovkina Cave in the Caucasus Mountains, which reaches a depth of about 2.2 miles (3.5 kilometers). Even this depth is a tiny fraction of the total thickness of the Earth's crust.
While caves can be incredibly deep, they are limited by the nature of the Earth's crust and the physical conditions that exist at great depths.
