The answer is subduction. In locations around the world, ocean crust subducts, or slides under, other pieces of Earth's crust. The boundary where the two plates meet is called a convergent boundary.

Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.

These convergent boundaries also occur where a plate of ocean dives, in a process called subduction, under a landmass. As the overlying plate lifts up, it also forms mountain ranges. In addition ...

Along convergent boundaries, subduction carries plates into the mantle; the material lost is roughly balanced by the formation of new (oceanic) crust along divergent margins by seafloor spreading.

For more on divergent boundary, go to The Sea Floor Spread. Also known as subduction boundary, a convergent boundary occurs where one plate slides under another as the two are pushed together.