Here are some hints to navigating the Underdark.
Some helpful tips:
1.) If you enter the Underdark near Fort Scinia, you find a campsite at the entrance "inside" the cave. However the cave is not just a cave as most of you have discovered. It is an immense 150' tall cavernous tunnel. This entrance is significant in description because the door is a "black oblivion door".
2.) Going "down" the Cavernous tunnel may seem endless. While traveling downward through the tunnel, at some point you will pass http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot30.jpg on the left, reach another campsite halfway down the tunnel, come to a fork in the tunnel (the left is the way) where spiders wait both ways, pass a goblin connection tribe on the right and finally reach http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot31.jpg that seems to lead to another huge cavern in the distance.
3.) That huge cavern in the distance is an illusion "in a sense". It is actually the invisible door transition that leads to the next cell. Uh huh, I said cell and it's larger too. No door? Well just head straight for it and you will reach the entrance to the Main Cavern.
4.) The Main Cavern has http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot34.jpg in the center of the this big place. There are several ways to go to different locations. If you immediately head to your left and you'll find a giant red meat eater

and http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot33.jpg where the great pit and Ched Nasad is located.
http://www.youtube.com/watch?v=5pmkYlT1QKg
If you go to your right while in the Main Cavern, there is the beginning of a closed sectional of http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot35.jpg. North of there is the entrance to the http://i283.photobucket.com/albums/kk286/aoikani/ScreenShot36.jpg.
5.) When finding the Northdark, walking straight will lead you to the great pit. If you head left after entering this cell of the Northdark, you'll find the area of the large vine at the far wall. While in this cell, on the opposite side of the great pit (the Kou-tan are at the bottom), you'll find the drider area and the entrance to Menzoberranzan.