Why does a mountain range run down the middle of the Atlantic?

A mountain range runs down the middle of the Atlantic because the ocean floor is being pulled apart there. The Mid-Atlantic Ridge is the seam between tectonic plates: North America on one side, Europe and Africa on the other in the north, South America and Africa in the south. Along its crest runs a deep rift valley where magma from the mantle rises, erupts as lava and cools into brand-new seafloor. The fresh rock piles up into a long ridge, and the plates on either side move apart at an average of about 2.5 centimetres a year.

It's part of the longest mountain range in the world. Mid-ocean ridges link up across every ocean into a continuous chain about 65,000 kilometres long, several times longer than the Andes, yet almost all of it is hidden under the sea. Sailors found it by accident. In 1872 the scientists aboard HMS Challenger, surveying a route for a transatlantic telegraph cable, discovered a great rise in the middle of the ocean. In the 1950s, the maps of Marie Tharp and Bruce Heezen revealed its central rift, a key clue that led to the theory of plate tectonics.

In a few places the ridge breaks the surface, most spectacularly in Iceland. At Þingvellir National Park you can walk between two plates, and in the Silfra fissure, filled with clear glacier water, divers and snorkellers float in the crack between North America and Eurasia, which widens by about 2 centimetres every year.