The Tubbataha Reef in the Sulu Sea is a marine sanctuary protected as the Tubbataha Reef National Marine Park. The reef is made up of two atolls, North Atoll and South Atoll, separated by a deep channel of approximately 5 miles (8 km) wide. It has become one of the most popular dive sites in the Philippines because of its coral walls where the shallow coral reef abruptly ends giving way to great depths. The marine park is open to live-aboard diving excursions between the months of April to June when the waves are most calm.