Deep Shroom

Found only deep inside caverns and mines, deep shrooms are a mushroom type that is loved by miners, spelunkers and explorers alike. Raw it already has a nice hearty taste, while turned into a stew it even has small beneficial effects, on top of being a great alternative to boring dry rations. On the downside, it is so delicious that it is easy to accidentally wipe out the entire population in an area.
Of course a deep shroom also means one is very deep underground, with all the dangers involved. So its presence also can represent despair instead of delight, as it illustrates exactly how far from civilization one really is. Thus the deep shroom is a symbol with conflicting meanings to those encountering it. A temporary relief in darkness.

Type

Fungi

Size

Bite-sized to hand-sized

Location

Deep caverns

Nutritional value

Excellent

Properties

Deep shrooms are gray or black, with the colour depending on age and environment. This colour seems to partially come from the coal residue that they absorb. Their first growth stage only takes a few days, after which they stop growing but occasionally spread spores.
When fertilized, they start a second growth stage that lets them reach the size of a large hand. This comes with growing lighter gray. As this means the shroom's taste becomes less pronounced, many prefer it this way despite the origin of the growth. Miners tend to make sure to keep nearby populations well-fertilized.
Asides from being quite nutritional, when prepared properly together with luminescent moss, the shroom can temporarily increase one's vision. This too helps its popularity amongst miners.

The deep shroom is extremely sensitive to light, to the point that a torch or lantern nearby can cause them to die. Yet at the same time they do seem to enjoy having a bit of light nearby, growing mostly in the neighbourhood of luminescent moss. It also feeds on coal residue in the soil. This means a population may die out if they run out of coal after overconsumption, so miners will often provide coal powder as additional fertilizer.
As a result of the sensitivity and coal requirements, deep shrooms are almost solely found deep underground. The rare exception is in coal storages, likely caused by spores deposited on mined coal.
While many have tried, only a limited few have succeeded in growing a population above ground. It appears they are very finicky soil-wise.