Ice caves are formed when meltwater runs under or through a glacier, melting the ice and leaving behind a cavern or passageways within the glacier. During the summer months, warmer temperatures and sunlight will melt the ice at the surface of the glacier. This meltwater drains downwards through crevasses in the ice or into sinkholes and moulins. These shafts are often almost vertical, allowing water to descend to the bottom of the glacier. The channels or conduits are far larger than most people who have never seen a glacier would imagine. They can be as wide as 10 meters and descend to the base of the glacier, helping it to move. In Iceland, many ice caves are formed by a combination of surface meltwater drainage and subglacial flows caused by geothermal warmth. Warm water flows or hydrothermal rivers will also, sometimes, cause ice caves to form. For example, there is one particular ice cave in Kverkfjöll which is wholly formed by geothermal activity but this is very difficult to access. The first type of ice cave is formed by meltwater streams carving labyrinths in the bases of glaciers or by streams and wind hollowing out tunnels in snowfields. These usually have scalloped, translucent walls that transmit a blue light.
The second type of ice cave occurs either when frigid winter air settles into downward-leading caverns where it cannot be forced out or when moisture freezes in currents of cold air. Frozen lakes, icicles, and ice draperies are common formations. Helictite-like icicles also form where air currents deflect the freezing water.