Nether-caps are a type of mushroom-like subterranean tree. Once fully grown, they can be designated for wood cutting and produce nether-cap logs. They often grow on muddied rock in the 3rd underground cavern layer and, once discovered, will begin to grow in any suitable environment within your fortress as well.
Nether-caps and their products are dark indigo, and they have the unique property of being naturally cold, having a fixed temperature of 10000 °U (the freezing point of water). This fixed temperature makes nether-cap wood magma-safe, except that nether-cap wood items dumped into magma are destroyed. To clarify, this means that you can use nether-cap as a substitution for metal magma-safe components (for example, in a magma pump stack). Interestingly, nether-cap logs can still be burned into ash or charcoal at a wood furnace. Nether-cap cannot cause water to turn into ice, since water only becomes ice when cooled below its freezing point.
Some dwarves like nether-caps for "their coldness to the touch".
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