Lake Wai'au, the third-highest lake in the United States at 13,020 feet on Mauna Kea. This lake isn't more than 10 feet deep, and is one of a kind. It is the only tropical lake in the world fed by permafrost. During the last ice age, a large glacier spread 3,000 feet down Mauna Kea. It left 350 feet of permafrost below the surface, and this is the mysterious source of this lake. 8-25-05.