These caves are formed almost entirely from steam fumaroles that send heat upward to bore out huge, cavernous rooms in steep passages. The eruption in 1980 created a vast crater with a north-facing breach - a virtual glacier factory. The smaller 2004 eruption reshaped the glacier and spawned a new generation of glacier caves.
This is an extremely young, active and dynamic sub-glacial environment, perfect for advanced studies in climatology, geothermal activity and microbiology.
For more on this project, watch this Oregon Field Guide show:
Discovery on Mt St Helens