The atoll forms a nearly complete rim of land around a lagoon, enclosing 90 percent of its perimeter, with an opening only in the north. The main island is the largest of about sixty islands which form the Chagos Archipelago. Besides the main island, there are three small islets at the mouth of the lagoon:

1. West Island (3.4 ha/8.4 acres)
2. Middle Island (6 ha/14.8 acres)
3. East Island (11.75 ha/29 acres)

A fourth island shown on some maps, Anniversary Island one kilometre southwest of Middle Island, appears as just a sand bar on satellite images. Both Middle Island and Anniversary Island sit on the Spur Reef complex.

The total area of the atoll is 174 km2 (66 mi2) according to [1], of which 30 km2 (12 mi2) are land, 17 km2 (6.5 mi2) peripheral reef and 124 km2 (48 mi2) are lagoon.


Annual rainfall averages 260 cm (102 in), with the heaviest precipitation from October to February. August, the driest month, averages 100 mm (4.2 in). Temperatures are generally close to 30°C (86 °F) by day, falling to the low 20s °C (70 °F) by night. Humidity is high throughout the year. The almost constant breeze keeps conditions reasonably comfortable.

Diego Garcia is at risk from tropical cyclones. The surrounding topography is low and does not provide an extensive wind break. Since the 1960s the island has not been seriously affected by a severe tropical cyclone, even though it has often been threatened. The maximum sustained wind associated with a tropical cyclone in the period 1970-2000 was approximately 40 knots (75 km/h).

The island was somewhat affected by the tsunami caused by the 2004 Indian Ocean earthquake. Service personnel on the western arm of the island reported only a minor increase in wave activity. The island was protected to a large degree by its favourable ocean topography. About 80 km (50 mi) east of the atoll lies the 650 km (400-mile) long Chagos Trench, an underwater canyon plunging more than 4,900 m (16,000 ft). The depth of the trench and its grade to the atoll's slope and shelf shore makes it more difficult for substantial tsunami waves to build before passing the atoll from the east. In addition, near shore coral reefs and an algal platform may have dissipated much of the waves' impact. A biological survey conducted in early 2005 indicated erosional effects of the tsunami wave on Diego Garcia and other islands of the Chagos Archipelago. One 200 to 300 m stretch of shoreline was found to have been breached by the tsunami wave, representing approximately 10 percent of the eastern arm. A biological survey by the Chagos Conservation Trust reported that the resulting inundation additionally washed away shoreline shrubs and small to medium size coconut palms.

On November 30, 1983, a magnitude 7 earthquake 55 km (34 mi) northwest of the island caused a small tsunami resulting in a 1.5 m (5 ft) rise in wave height in the lagoon, causing some damage to buildings, piers and the runway. Immediately following the earthquake, many of the military and civilian residents of the island gathered at the Naval Support Facility swimming pool. The hill built to enclose the swimming pool, at 22 feet above sea level, is the highest point on the island.