Ice caves form where sub-zero temperatures persist year-round inside glaciers, lava tubes, or bedrock caverns, letting ice formations survive long after the surrounding season changes. Glacial ice caves like those under Iceland's Vatnajökull are carved by seasonal meltwater tunneling along fracture lines, and often only stay open in winter once meltwater flow slows enough for the passages to remain stable. Ice looks blue because dense ice absorbs red wavelengths of light while scattering blue, an effect that intensifies with ice thickness and compaction. Interior cave temperatures typically hover close to 0°C, a delicate balance that lets stalactite-like ice formations grow through repeated cycles of sublimation and refreezing, building the crystal spires and hoarfrost seen here.
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