An slope of Green Mountain was covered by concrete to help collect as much of the sparse rainfall as possible. Ascension Island is a volcanic island in the equatorial Atlantic ocean, 1,600 kms from the coast of Africa and 2,250 km from the coast of South America. It is home to RAF Ascension Island, a British naval base, the Atlantic relay station for the BBC's World Service and one of the five antennae that are instrumental in the Global Positioning System (GPS) navigation system. In the 1850, with encouragement from Charles Darwin and under the auspices of Joseph Hooker, a botanist and director of the Royal Botanical Gardens at Kew, the Royal Navy was encouraged to import various types of trees to the island. They flourished and by the 1870s enough trees had grown on the highest peak, Green Mountain, to create a cloud forest which improved the islands fertility.