Mountains aren't just big piles of dirt, they're made of solid rock. Believe it or not, the rocks that make up the Himalayan mountains used to be an ancient sea floor. Over millions of years, rivers washed rocks and soil from existing mountains on the Indian subcontinent and nearby Asia into a shallow sea where the sediment was deposited on the floor. Layer upon layer of sediment built up over millions of years until the pressure and weight of the overlying sediment caused the stuff way down deep to turn into rock. Then about 40 million years ago, in a process called "uplifting", the sea floor began to be forced upward forming mountains.