Lake Waiau is the highest lake on the Big Island, the state of Hawai'i, and the entire Pacific Rim. The Big Island's only other lake, Green Lake, is located on private land in the Puna District. This shallow lake's existence is a mystery. Although there are a few theories, scientitsts are not sure how a lake can exist in such an arid, high altitude location on an island with very porous ground. For some unknown reason, water collects in the basin created by the erosion of Pu'u Waiau. The reason why it does not percolate so easily into the lava rock like water does throughout the rest of the island is unknown. Two theories attempt an explanation: either sulfurous steam altered the volcanic ash to low-permeability clay soil; or explosive interactions between rising magma and water (phreatic eruptions) formed exceptionally fine ash that also would reduce the permeability of the lake bed. One colorful explanation holds that permafrost from the last ice age feeds the lake, altough no permafrost has been found in the area around Pu'u Waiau. Permafrost has been found closer to the summit of Mauna Kea in a few areas.