In some areas of the Underdark, phosphorescent mushrooms and phosphorescent lichens provide enough illumination for light as bright as a torch or even a lantern. They grow in pockets in every layer of the Underdark.
The phosphorescent mushrooms vary in their sizes: some smaller than a finger, others taller than a human. The amount of light shed by each varies by their size. A small mushroom provides the light of a candle, but because they can grow in large clusters and colonies, they can collectively brightly light an entire cavern. Their hue also varies: many varieties shed a warm yellow-orange light, but different varieties can radiate shades of blues, pinks, violets, greens, and other shades.
Many varieties of phosphorescent fungi are edible and nourishing, but some are quite poisonous. Those who aren’t native to the Underdark can make a Survival check (DC 18) to determine which are safe or dangerous. Those with specialized Underdark training, such as Druids of the Circle of Spores or Circle of the Land with an Underdark specialty, or Rangers with the Underdark as a favored terrain, automatically succeed in their check.
It is nearly impossible to cultivate phosphorescent mushroom outside of the Underdark, as they are hypersensitive to sunlight. They take 1d10 radiant damage each round they are exposed to normal sunlight, and can take only 10 hit points of damage from sunlight or radiant damage before they die. The dead mushrooms lose their luminescence within a minute, but they remain otherwise intact and nontoxic varieties remain edible.
Sages and scholars debate on whether the phosphorescence gives the mushrooms an advantage or disadvantage. Some believe that the light can frighten away lower-intelligent creatures native to the Underdark, particularly those with sunlight sensitivity or hypersensitivity. Others believe that the light is meant to attract predators, arguing that the mushrooms propagate and spread when their undigested spores are later eliminated from the body.
Some subterranean civilizations like the Drow are known to cultivate phosphorescent fungi in public areas. Even though these creatures have darkvision, that sight is limited to shades of gray, and there are areas where the ability to see colors is appreciated, such as areas with civic art, dramatic performances, or fighting pits and gladiator arenas. Some artists use the mushrooms themselves as art, cultivating different varieties growing together to form patterns of light.