The longest mountain range on Earth is not where you might expect to see it. The Mid-Atlantic Ridge is a 10,000 km long underwater chain of mountains. It runs down the middle of the deepest part of the Atlantic Ocean. The ridge is about 3.2 km above the seafloor, which has an average depth of 4.8 km.

The theory of plate tectonics explains how this mountain range was formed. There are two main plates beneath the ocean. Each plate is moving away from the center toward the landmasses that border the ocean. As the plates separate, the ocean floor spreads, and magma rises from the mantle to fill the crack in the ocean floor. Billions of years of crustal movement have caused layers of magma to rise to the surface of the ocean floor and then cool.