An Ice Cave is a cave which contains ice, not a cave formed inside ice those are called Glacier Caves. Ice caves are also called Glacieres or freezing caverns.
Ice caves may be caves of any type, karst caves, primary caves like lava tubes or tectonic caves. What they have in common is the fact, that there is ice inside the cave. The ice is formed by dripping water and the low temperature of the cave itself. The resulting ice structure look like stalactites and stalagmites, like ice lakes or glaciers.
But it is not that easy: If the cave temperature is below zero, why did the water not freeze inside the rock? The temperature of the rock must be above 0°C, the cave air below zero. And if this is the case, why ist the cave not completely filled with ice? There must be a change, and a period of ice accumulation is followed by a period of melting ice. So there are two mechanisms working together:
1. the shape of the cave works as a trap for cold air.
Very often, an ice cave has a special shape, a high entrance and no exit at the bottom. The entrance allows cold air, which is heavier than warm air, to flow down into the cave. So the cave temperature gets colder than the temperature of the rocks. In summer the cold air stay inside the cave because of its higher specific weight. It is the same principle used in supermarket refrigerators to keep the cold inside.
2. the average temperature outside is a bit above 0°C and there are warm and cold seasons.
The average long term temperature outside is also the temperature of the rocks and normally that of the cave. If there are cold winters, there is cold air to catch in the "temperature trap". But if the average temperature is below zero the cracks are blocked by ice and there is no water.
Ice caves are very cold, always below 0°C, the paths are often hard to walk on. It is normally not possible to build concrete paths in the areas with ice, as the ice moves every year and tends to destroy anything in its way. The paths across the ice are often rebuild every year using wooden planks and ropes. Ice caves often have no electric light, as the same problems with moving ice apply to electric installations. Visitors are equipped with headlamps or carbide lamps, the guide additionally use magnesia ribbon, which produces a very bright light when it burns down.
It is essential to wear warm clothes and good shoes. The cave is not suitable for people with health problems.