The black rock arches and lava tubes are indicative of the volcanic origin of the islands. The tunnels are a part of Thatch Cay, an island northwest of St. Thomas. Bright cup corals and sponges are visible from the moment of entry. Divers wind their way through a series of tunnels, past big boulders and gorgonian patches actually swimming though the island. Butterflyfish, parrotfish and trumpetfish spend their days in the gorgonians, while tarpon pursue schools of silversides. Moray eels and spiny lobsters are regular residents of the reef.