Theory describing the mass balance in the Earth's crust , which treats all large portions of the crust as though they were floating on a denser underlying layer, about 70 mi (110 km) below the surface.
In this theory, a mass above sea level is supported below sea level, so high mountains must be regions where the crust is very thick, with deep roots extending into the mantle . This is analogous to an iceberg floating on water, in which the greater part of the iceberg is under water.