The Turks and Caicos Islands are famous for the reef system that surrounds them, the third most extensive coral reef in the world, making it a prime destination for divers and snorkellers. Visibility and an abundance of marine life, as well as excellent dive facilities ensure that diving is the number one activity on the islands. Coral reefs, particularly on the western shores, provide some outstanding vertical walls, such as the Grand Turk Wall west of Grand Turk that drops from 20ft (6m) to over 6,562ft (2,000m). The channel that separates the Turks Islands from the Caicos Islands is a deep passage that is a major route for humpback whales, dolphins, and eagle and manta rays. The reef wall around the southern edge of South Caicos Island is supposed to be the finest diving in the archipelago, and is known as the big fish capital. The shallow reefs close to the shore are excellent for snorkellers and with visibility up to 200ft (61m) there is no need to be underwater to appreciate all the sights.