How do you go to the under city in knights of the old republic?

Answer 1

Once your in the lower city sweet talk your way into the Hidden Bek Base (no persuasion skill required, but you can't enter if you are wearing your Sith uniform). Once inside make your way over to Gadon (he's standing behind a table with a twi lek bodyguard), talk to him and agree to trade your Sith uniform for security papers to pass into the Under City (don't worry with the papers you can still go back into the lower city and upper city).