Mt. Rainier has a magma chamber, or reservoir of molten rock, deep underneath it. During eruptions, this magma pushes upwards through the volcano and expels onto the surface, leaving the chamber empty. Forces like the weight of the glaciers and seismic landslides creating weak spots on the outside of the volcano leads to the magma chamber collapsing, leaving huge craters.

Volcanoes can erupt with terrifying force, sometimes blasting large portions of rock and debris off their summits as buildups of gas and lava push upwards and explode outwards at the top, where there is less pressure and weight. These explosions leave smaller (but still very large) craters at the top of the volcano.

One of Mount Rainier's most unique features lies hidden underneath sheets of ice and snow, buried inside the glaciers. Heat and steam from fumarole vents covering the craters and surrounding areas have, over thousands of years, melted away sections of glacial ice, carving a hidden labyrinth of ice caves and tunnels underneath the glaciers. These tunnels can be dangerous, due to toxic gases coming from fumarole vents, but section are open for visitors to explore this beautiful feature.

An spelunker exploring the ice caves under a Mt. Rainier glacier.