The Mid-Atlantic Ridge is a huge underwater mountain range that runs from just 207 miles (333 km) south of the North Pole to Bouvet Island, located in the Atlantic ocean at a longitude just below South Africa. At about 6,200 miles (10,000 km) long, this range connects to a series of oceanic ridges that encircle the entire planet, with a total length of about 25,000 miles (40,000 km). The mountains are caused by seafloor spreading, the volcanic activity that happens in the central areas of oceanic crust to replenish crust lost through subduction (sliding) under contintental plates. As the subduction only occurs at a rate of only about 1 inch (2.5 cm) a year, only modest volcanic activity is necessary to replenish the lost crust, though over geological time it can create great mountain ranges.
Although the existence of the Mid-Atlantic Ridge was first inferred by Matthew Fontaine Maury in 1850, it was not discovered until 1872, when an expedition of the HMS Challenger discovered the ridge while scouting for a transatlantic telegraph cable. In 1925, a confirmation of the ridge, as well as additional details, were uncovered by sonar. It was found to be 300 to 600 miles (482.8 to 965.6 km) wide, consisting of a parallel series of ridges increasing in height. The mountains are about 2 miles (3.2 km) above the sea floor, which has an average depth of 3 miles (4.8 km). The ridge serves to divide the Atlantic sea floor into two distinct basins.
Some peaks of the Mid-Atlantic Ridge are so tall that they rise above the waterline to form islands. These include Jan Mayen in the Arctic Ocean, Iceland, the Azores, Bermuda (which initially formed on the ridge but is now far west), the Saint Peter and Saint Paul Islets, Ascension Island, Tristan da Cunha (which has the highest point on the ridge, Queen Mary's Peak, 1.24 miles (2 km) above sea level), Gough Island, and Bouvet Island. Many of these islands are inhabited. The range is not a continuous line, but rather consists of many sections misaligned with the others, due to millions of years of intense geological activity.