Guam (Chamorro: Guåhån) is an island in the Western Pacific Ocean, at the southern end of the Mariana Islands chain. It lies between 13.2°N and 13.7°N and between 144.6°E and 145.0°E. With an area of 209 square miles (541 km²), it is the largest island in Micronesia.

Guam and the rest of the Mariana Islands were all created by the colliding Pacific and Philippine tectonic plates. This has also created the Marianas Trench, a deep subduction zone to the east of the islands, which contains the deepest surveyed point in the world ocean (35,797 feet / 10,911 m) deep. The highest point in Guam is Mount Lamlam (1,332 feet / 406 m). The island of Guam is 30 miles (48 km) long and 4 mi (6 km) to 12 mi (19 km) wide. Guam is not volcanically active but it does experience occasional earthquakes.

Guam is divided by a prominent geologic fault (Pago-Adelup fault) into two geologically, topographically, hydrologically, and environmentally distinct halves.

The northern part of Guam is a forested coralline limestone plateau, while the southern part contains volcanic peaks covered in forest and grassland. The two halves of the island are environmentally quite different.

The southern part of Guam is comprised primarily of dissected, relatively impermeable, volcanic formations. They support many streams and surface water reservoirs. A ridge of high ground runs north-south close to the western coast. The slope of terrain is very steep from the ridgeline to the western coast; from the ridgeline toward the eastern coast the slope is more gradual.

This on-line atlas is focused on southern Guam.