These include four main islands: Grande Comore, Anjouan and Mohéli, of the Union of the Comoros, and Mayotte, a department and region of France.

Grande Terre has a deeply indented coast with many bays, peninsulas, and rocky headlands, and is home to most of the Comoro Islands' mangroves.

[3] Mohéli (Mwali) has rugged terrain, with an east–west ridge running the length of the island and many small stream valleys.

The island has an extensive fringing reef along the south shore, and eight small islets rise steeply from the lagoon.

The young volcanic soils on Grande Comore are very porous, and rain percolates rapidly into the ground instead of creating permanent rivers and streams.

[1][3] Formerly the natural vegetation on the islands was evergreen tropical moist forests, which extended from sea level to 1800 meters elevation.

Characteristic trees include Ocotea comorensis, Khaya madagascariensis, Olea capensis, Gambeya boiviniana, Prunus africana, and Filicium decipiens.

[1] Stunted trees and heathlands, dominated by the giant heath Erica comorensis, grow between 1800 and 2200 meters on Mount Karthala, Grande Comore's central volcanic peak.

Dry grasslands, including species of Poa, Agrostis, and Festuca, replace the heathlands in areas cleared by recent fires.

[4] There are more birds, mammals, and reptiles than one would expect to find on an Indian Ocean island, including lemurs, as in nearby Madagascar.

[9] Protected areas on Mayotte include Pointes et plages de Saziley et Charifou and Ilôt Mbouzi National Nature Reserve.

[10][11][12] Grande Comore, dominated by Mount Karthala, which can be climbed in one or two days, is the largest and most developed island and location of the international airport.

Anjouan is reached by boat from Grande Comore, the two features here are Mount Ntringui and Lake Dzialandzé, both Ramsar List sites.

Moheli is the smallest and quietest island and the Mohéli National Park, home to sea turtles, dolphins and whales, is located on five offshore islets.