Minecraft:Underwater Torch

Obtaining

Underwater torch is immediately destroyed when broken. Template:Breaking row Underwater torch drops itself when it's destroyed.

The underwater torch can be used exactly like a normal torch, only it can be underwater.

Underwater torches can be placed on the top or the sides of most solid blocks, although some require Template:Control (see placement for exceptions). Being non-solid, an underwater torch has no collision box.
Gravity-affected blocks like Minecraft:sand and Minecraft:gravel do not fall if the block below them has an underwater torch on it, and they break if they fall onto an underwater torch.

In general, underwater torches can be placed only on solid, opaque, and full blocks; exceptions are listed below.

Underwater torches emit a Minecraft:light level of 14. Underwater torches also melt Minecraft:snow layers and Minecraft:ice within 2 blocks (Minecraft:taxicab distance).

Underwater torch is destroyed when the Minecraft:piston tries to push it. It can't be pulled by the Minecraft:sticky piston.

The crafting recipe refers to the fact that magnesium metal can burn in pure water vapor. Magnesium has a similar reaction with hot water, but it is slower and does not look like burning.
If magnesium catches fire, water cannot be used to extinguish it. This is because magnesium reacts with hot water (and water vapor) to form explosive hydrogen gas.