when was this discovered? Who told you this?
What about very tall hills, or just ordinary hills?
The Himalayas are folded sedimentary deposits - you can see the folds, they don't have depth unless you mean the underlying bedrock (different from the folded material) which extends beneath areas with no mountains.

I don't go to mythical places with strange men.
-- Douglas Adams, The Long Dark Tea-Time of the Soul.