On the western side of Mount Hood lies the longest glacier cave system in the contiguous United States. In 2012, these caves were mapped to a combined length of 7,166.8 feet by cave explorers Brent McGregor and Eddy Cartaya. Currently, the total passage length is hundreds of feet less. Glaciers are frozen rivers; they are always moving and changing. In the past five years, we have seen the caves melt, shrink and collapse in a dramatic way. The caves are formed by water carving away at the ice. Surface water makes its way downward through a fracture in the glacier and melts away a vertical shaft called a moulin. The water then travels down the moulin and downhill, thus creating a horizontal borehole passage. There are three explored caves in the glacier: Snow Dragon Cave, Pure Imagination and Frozen Minotaur. There is also one cave that has not yet been explored. Here: A large frozen column is backlit by sunlight beaming through the entrance in Pure Imagination Cave. (Photo and caption by Josh Hydeman)

A caver gives scale to the entrance to Snow Dragon Cave. (Photo and caption by Josh Hydeman)

A caver stands in the entrance of Pure Imagination Cave. The moulin on the left hand side of the image was pouring down water and acting as a funnel for debris and rocks to shoot through. The cave was in a very wet and active state during this time. (Photo and caption by Josh Hydeman)

A view of Mount Hood in Autumn. You can see where the entrances of the glacier caves are at the bottom of the glacier. It is interesting to see how much rockfall covers the glacier in the summer. (Photo and caption by Josh Hydeman)

Traditionally it is frowned upon to camp in caves but in winter conditions like this it is very helpful to stay warm. (Photo and caption by Josh Hydeman)

A caver examines the beautifully scalloped walls of ice in Pure Imagination Cave. (Photo and caption by Josh Hydeman)

Traditionally it is frowned upon to camp in caves but in winter conditions like this it is very helpful to stay warm. (Photo and caption by Josh Hydeman)

Frozen waterfalls guard the entrance to the Snow Dragon Cave. (Photo and caption by Josh Hydeman)

Cavers wake up and leave their tents as the sun rises in Snow Dragon Cave. (Photo and caption by Josh Hydeman)