The chimney in the mountain
Eisriesenwelt, "World of the Ice Giants" in German, is the world's largest ice cave, with a surveyed length of over 42 kilometers. It is a dynamic cave; its its internal climate is actively driven by the temperature difference between the inside and the outside. The cave system has openings at different elevations within the Hochkogel mountain, which creates a natural ventilation system known as the "chimney effect."
In winter, the air inside the mountain is warmer and less dense than the frigid air outside. Cold, heavy air flows down into the lower cave entrances, displacing the warmer air which rises and exits through higher openings. The process chills the lower sections of the cave to below 0°C. In spring, meltwater from snow seeps through cracks in the limestone. When this water reaches the supercooled parts of the cave, it freezes, forming the immense ice structures.
During summer, the opposite occurs. The air outside is warmer than the cold, dense air settled inside the cave. This heavy internal air effectively creates a "cold air dam," preventing the warmer air from penetrating deeply. A cold wind blows out of the cave entrance. Any warmer air that does enter is rapidly cooled, and its moisture condenses and freezes, adding new layers to the ice formations. The ice therefore appears to grow in the summer.
Melting under pressure
The weight of the ice itself introduces another physical process. Some ice formations, like the Great Ice Embankment, reach a thickness of 25 meters. This enormous mass exerts high pressure on the ice layers at the very bottom. Water is an unusual substance; unlike most materials, its melting point decreases as pressure increases. At the base of these thick ice formations, the pressure is sufficient to lower the melting point to just below the ambient cave temperature.
This phenomenon, known as pressure melting, causes the ice to melt at its base even when the cave's air temperature is below freezing. The resulting meltwater can then flow to other parts of the cave and refreeze, adding to the dynamic and ever-changing nature of the cave. Analysis of the ice layers suggests the oldest ice in the cave is approximately 1,000 years old. The cave itself began forming around 100 million years ago as the Salzach river and millennia of water erosion carved passageways through the limestone of the Tennengebirge mountains.