readily absorb heat. Although the portion of a cave within this permafrost zone will be below freezing, permafrost generally does not allow water percolation, so ice formations are often limited to crystals from vapor, and deeper cave passages may be arid and completely ice-free. Ice caves in permafrost need not be cold-traps (although some are), provided they do not draught significantly in summer. Evaporative cooling: In winter, dry surface air entering a moisture-saturated cave may have an additional cooling effect due to the latent heat of evaporation. This may create a zone within the cave that is cooler than the rest of the cave. Because many caves have seasonally-reversing draughts, the corresponding warming of the cave through condensation in summer may occur at a different location within the cave, but in any event a moisture-saturated cave environment is likely to experience much more evaporative cooling than condensative warming. Types of ice Different freezing mechanisms result in visually and structurally distinct types of perennial cave ice. Ponded water: Surface water that collects and ponds in a cave before freezing will form a clear ice mass, and can be tens of metres thick and of great age. Large ice masses are plastic and can slowly flow in response to gravity or pressure from further accumulations. Sculpting from air flow and sublimation may reveal ancient accumulation bands within the ice. Accumulated snow: Compressed under the weight of ongoing accumulations, snow sliding or falling into a cave entrance may eventually form ice that is coarsely crystalline, akin to glacier ice. True underground glaciers are rare. Ice formations: Water that freezes before ponding may form icicles, ice-stalagmites, ice columns or frozen waterfalls. Airborne moisture (water vapor): Freezing vapor can form frost crystals, frost feathers and two-dimensional ice plates on the cave walls and ceiling. Needle ice: Infiltrating water that freezes within the bedrock can sometimes be forced into the cave passage. Intrusions: The weight of a surface glacier perched atop a cave entrance can force glacial ice a short distance into the cave. The only known examples of this phenomenon are the several 'ice plugs' at the back of Castleguard Cave in Alberta. Examples Bandera Volcano Ice Cave (New Mexico, USA) Bixby State Preserve (Iowa, USA) Booming Ice Chasm (Alberta, Canada) Bortig Pit Cave (Apuseni Mountains, Romania) Canyon Creek Ice Cave (Alberta, Canada) Castleguard Cave (Alberta, Canada) Coudersport Ice Mine (Pennsylvania, USA) Decorah Ice Cave