Sandy Glacier Caves were a changing system of glacier caves inside Sandy Glacier on Mount Hood, formed by internal melting near the Muddy Fork River headwaters. Regarded as the largest glacier caves in the lower 48 states, they measured more than a mile of passages in surveys from 2011 and 2012. Seasonal melting alternately linked and separated caves known as Snow Dragon, Pure Imagination, and Frozen Minotaur, while explorers and National Speleological Society members mapped them and brought scientists into the glacier. The caves offered a striking record of a glacier in retreat before collapse and rapid disappearance by 2016, chiefly linked to climate change.