Lake Waiau, Sacred lake at 3,970 meters on Mauna Kea, Hawaii, United States.

Lake Waiau lies inside the Puʻu Waiau cinder cone on Mauna Kea at 3,970 m, about 100 m across and roughly heart-shaped. Its surface averages around 6,000 m² but shrinks to as little as a third of that by late summer, when depth falls below 1 m. The lake sits on an impermeable layer of unknown composition above 7.5 m of sediment.

The lake has been regarded as a sacred place by Hawaiians for many generations, long before outside explorers arrived on the island. In the 19th century, a member of the Hawaiian royal family visited the lake during an expedition to the mountain, marking one of the earliest recorded outside accounts of the site.

The lake is named after Waiau, a snow goddess associated with Mauna Kea who, according to Westervelt, used to bathe in it. The whole summit area, including the lake, was considered sacred, and only priests and chieftains were allowed access. After the formation of the Hawaiian kingdom, members of the royal family occasionally visited; Queen Emma bathed in the lake in 1881.

An official hiking trail now runs from the summit road to the lake, so no four-wheel-drive detour is needed beyond reaching the trailhead. The lake sits at 3,970 m, where the air is thin and the water often freezes in winter; allow time to acclimatise at lower elevations and bring warm, windproof clothing.

Lake Waiau is one of the highest lakes in the United States, sitting above most peaks on the continental mainland. Scientists have not fully explained how the lake retains water year-round in one of the driest summit zones on Earth, since the porous volcanic rock around it should drain any moisture away quickly.