Lake Lanao is formed by the tectonic-volcanic damming of a basin between two mountain ranges and the collapse of a large volcano. It has a maximum depth of 400 feet (122 meters) and a mean depth of 198 feet (60.3 meters). The basin is shallowest toward the north and progressively gets deeper toward the south.

The lake, second largest in the Philippines after Laguna de Bay, was proclaimed as watershed reservation in 1992 through Presidential Proclamation 971 to ensure protection of forest cover and water yield for hydropower, irrigation and domestic use. It is counted as one of the 15 ancient lakes in the world.