South Island is Lake Turkana's largest island. Situated in the southern sector of the lake where the water reaches a depth of 374 feet, the island is a spine of volcanic cones and hills covering 15 square miles. The lava flows are remarkably well preserved. In 1955, this island was the last part of the lake to be explored. The remarkable colour of the lake's alkaline water is caused by green algae with high chlorophyll concentrations. People often refer to the lake as the Jade Sea.