The Cocos Keeling Islands are two coral atolls that have developed on top of old volcanic underwater mountains, rising from the depth of 5000 metres in the northeast Indian Ocean.

The islands' foundations are two of a series of undersea features known as the Vening Meinesz Seamounts. This undersea range of mountains also includes Christmas Island and extends in a north north-easterly direction from a prominent Indian Ocean sea floor feature known as the Ninetyeast Ridge. The Cocos atolls are two peaks in a section of the range known as the Cocos Rise and are connected by a narrow underwater bank at a depth of 700-800 metres.

Atolls are more or less circular coral reefs enclosing a lagoon, but without any land inside. On large atolls, parts of the reef have been built up by wave action and wind to form low island chains connected by the reef.