In most of the world, bedrock caves are thermally insulated from the surface and so commonly assume a near-constant temperature approximating the annual average temperature at the surface. In some cold environments, such as that surrounding Mount Erebus, average surface (and thus cave) temperatures are below freezing, and with surface water available in summer, ice caves are possible and are sometimes overlain by fumarolic ice towers. However, many ice caves exist in temperate climates, due to mechanisms that result in cave temperatures being colder than average surface temperatures where they formed.