Levels in the Ice Caves may contain platforms made of thin ice, usually suspended above spikes.
These will gradually melt away while you stand on them, dropping you onto whatever lies below, most often spikes or the Abyss.
The Giant Yeti's roar damages this kind of ice as well as dropping ice blocks.
Enemies and items may stand on the ice without causing it to melt. It is used as part of the area's Golden Idol trap.
Thin Ice cannot be found in Spelunky's Level Editor.