The entrance to the Underdark was investigated under a contract offered by the Baron.
The key features are as follows:
The Tunnel: This tunnel is approximately five miles in length with a descent of about a mile. It leads to the Ledge. Since it has been explored, this distance can be traversed in a day of hard travel.
The Ledge: The Ledge is an area about 1000 square yards that held supplies in the past. The cavern is well lit thanks to bioluminescent fungi that grows on the wall of the crevasse.
The Staircase: A long natural ramp that winds back and forth the side of the crevasse. It descends two miles with stops about about every mile in depth. It takes about eight hours to descend each mile following the trail.
The Base: A small ledge by an underground pond, currently a small tribe of Kua-Toa reside on the shores. The water is clear and cold and teeming with underground aquatic life. There is a dam and a stream flows deeper into the Underdark.
The Fungi Cave: A large cave on the road from the Base to the outpost of Bulwark. It is approximately a half mile by a half mile. A family of fungus people live here harvesting fungi and growing the cavern.
The Bulwark Crossroads: A crossroads into the Middle and Deep Underdark. A settlement of Mountain Dwarves and Druegar command this crossroad. The routes deeper into the Underdark are guarded by Bulwark.