Visitors to the Underdark who do not have darkvision are advised to bring artificial or magical sources of light in their subterranean travels, for they cannot rely on the rare forms of natural illumination underground.
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. Some subterranean civilizations are known to cultivate phosphorescent fungi in public areas. These creatures may have darkvision, but 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, or fighting pits and gladiator arenas.
The Underdark is also home to rare gemstones called Luminescent Crystals that naturally radiate light. Each one has the brightness of a candle, but their illumination magnifies when they are brought together in clusters. Dwarves and gnomes often trade these crystals with wealthy surface-dwellers who use them in chandeliers and wall sconces to magically light their castles at night.