mid-ocean ridge

When viewed from above, the wide-open sea is vast and flat. But venture below and you'll find a varied landscape that includes the deepest valleys and tallest mountains on Earth. These underwater mountains, or mid-ocean ridges, are formed from volcanic activity. They occur where two of the plates that make up the Earth's crust are moving apart. As the plates spread, magma rises up between the cracks and cools and hardens.

Image credits: main image, courtesy of AMNH; Rosamond Kinzler: AMNH.