Underdark
The Underdark is a many-layered network of caverns, passages, and rivers of water and magma. Its stone ceilings look down on strange features: stalagmites sharp as dragons’ teeth, lacy rock curtains, arches and graceful bridges. A heavy, brooding silence is broken only by far-borne echoes. They magnify sounds of life, cracks and deep boomings of shifting and shattering rock, and the murmur of moving water.
The Underdark is too dangerous for most beings to travel in alone. Its natives almost all possess infravision - and lurk in the magical darkness hunting those weaker than themselves.
Areas of the Underdark favored by the drow have large deposits of ferrous ore. Adamantite and gems seem to be plentiful. Magnetic forces are strong, and the rock gives off radiation akin to magical energy. Shiram, the magical radiation, distorts and interferes with certain types of magic. Being underground, wood is very rare and quite valuable to the drow.
Various fauna live in the different terrains of the Underdark. For example, the druegar live in the filth and garbage pits; the drow live in the large caverns; the caves along the walls and ceiling of the Underdark are inhabitted by beholders. Flora is more rare and usually consists of fungus and lichens. The fungus are used as food for many animals and the drow; and the lichens come in millions of shades and hues and are often used for decorative purposes.
The bulk of the Underdark can be accessed through the Darkwood Forest, though there are off-shoots of the Underdark beneath Indel and beneath the Barb Caves.