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.

Definition: a long underwater mountain range formed from volcanic activity
Location: stretches around the globe, through all the oceans
Length: 70,000 kilometers (43,500 miles)
Plate interaction: spreading (two plates moving apart)
Cool fact: In addition to volcanoes, earthquakes and faults are common along mid-ocean ridges.

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