The ground level on the islands never goes above three meters. The lagoons on the islands are warm and the saltwater lakes are easy to follow. Gaps in the reef leave narrow channels known as kandu, which give passage from the lagoon directly into the sea.
On sandy islands, rainwater collects just below the surface, which is very porous, and creates a surface of fresh water, which is easy to get to by way of shallow wells.
The northern and central atolls create a barrier that is narrowly passable by ships traveling across the Indian Ocean. The southern atolls are separated from the northern islands by two ocean gaps, which are wide and deep. These are the One and Half Degree Channel, which is between the Laamu and Huvadhu atolls, and the Equatorial Channel between the Huvadhu and Fua Malaku and Addu atolls. There are 400 km of deep ocean just south of the Addu atoll until the Chagos Archipelago.