FYI: You can re-enter the underdark in Chapter 6 SOA

Tutankhamen

A lot of on-line guides will tell you that once you're done with Chapter 5 you can't go back to the underdark. This is untrue. While you can't re-enter the underdark from the overworld map, You can still go back by going into the exit from the underdark area and walking back through the caves.

dunbar

Yup, I've tried stashing drow weapons there before in the hope that I'll be able to come up with a cunning plan to retrieve them at a later date.

borntodie

Wow, thanks! Nice to know.