The island of Bali lies 3.2 km (2 mi) east of Java, and is approximately 8 degrees south of the equator. Bali and Java are separated by Bali Strait. East to west, the island is approximately 153 km (95 mi) wide and spans approximately 112 km (69 mi) north to south; its land area is 5,632 km².
The highest point is Mount Agung at 3,142 m (9,426 feet) high, an active volcano that last erupted in March 1963. Mountains range from centre to the eastern side, with Mount Agung the easternmost peak. Mount Batur (1,717 m) is also still active; an eruption 30,000 years ago was one of the largest known volcanic events on Earth. In the south the land descends to form an alluvial plain, watered by shallow, north-south flowing rivers, drier in the dry season and overflowing during periods of heavy rain. The longest of these rivers, Ayung River, flows approximately 75 km.
The island is surrounded by coral reefs. Beaches in the south tend to have white sand while those in the north and west have black sand. The beach town of Padangbai in the south east has both. Bali has no major waterways, although the Ho River is navigable by small sampan boats. Black sand beaches between Pasut and Klatingdukuh are being developed for tourism, but apart from the seaside temple of Tanah Lot, they are not yet used for significant tourism.
The largest city is the provincial capital, Denpasar, near the southern coast. Its population is around 300,000. Bali's second-largest city is the old colonial capital, Singaraja, which is located on the north coast and is home to around 100,000 people. Other important cities include the beach resort, Kuta, which is practically part of Denpasar's urban area; and Ubud, which is north of Denpasar, and is known as the island's cultural centre.
Three small islands lie to the immediate south east and all are administratively part of the Klungkung regency of Bali: Nusa Penida, Nusa Lembongan and Nusa Ceningan. These islands are separated from Bali by the Badung Strait.
To the east, the Lombok Strait separates Bali from Lombok and marks the biogeographical division between the fauna of the Indomalayan ecozone and the distinctly different fauna of Australasia. The transition is known as the Wallace Line, named after Alfred Russel Wallace, who first proposed a transition zone between these two major biomes. When sea levels dropped during the Pleistocene ice age, Bali was connected to Java and Sumatra and to the mainland of Asia and shared the Asian fauna, but the deep water of the Lombok Strait continued to keep Lombok and the Lesser Sunda archipelago isolated.