Bali lies 3.2 km east of Java and approximately 8 degrees south of the equator. East to west, the island is approximately 153 km wide and 112 km north to south (95 by 69 miles, respectively), with a surface area of 5,632 km2. The highest point is Mount Agung at 3,142 m (10,308 feet) high, an active volcano that last erupted in March 1963. Mountains cover centre to the eastern side, with Mount Agung the easternmost peak. Mount Batur (1,717 m) is also still active. About 30,000 years ago it experienced a catastrophic eruption – one of the largest known volcanic events on Earth.
In the south the land descends to form an alluvial plain, watered by shallow rivers, drier in the dry season and overflowing during periods of heavy rain.
The principal cities are the northern port of Singaraja, the former colonial capital of Bali, and the present provincial capital and largest city, Denpasar, near the southern coast. The town of Ubud (north of Denpasar), with its art market, museums and galleries, is arguably the cultural center of Bali.
There are major coastal roads and roads that cross the island mainly north-south. Due to the mountainous terrain in the island’s center, the roads tend to follow the crests of the ridges across the mountains. There are no railway lines.
The island is surrounded by coral reefs. Beaches in the south tend to have white sand while those in the north and west black sand. The beach town of Padangbai in the south east has both: the main beach and the secret beach have white sand and the south beach and the blue lagoon have much darker sand. Pasut Beach, near Ho River and Pura Segara, is a quiet beach 14 km southwest of Tabanan. The Ho River is navigable by small sampan. Black sand beaches between Pasut and Klatingdukuh are being developed for tourism, but apart from the seaside temple of Tanah Lot, this is not yet a tourist area.
To the east, the Lombok Strait that separates Bali from Lombok marks the biogeographical division between the fauna of the Indomalayan ecozone and the distinctly different fauna of Australasia that is known as the Wallace Line, for Alfred Russel Wallace, who first remarked upon the distinction 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.