To get to Underworld, go out the exit on Three Dog's floor. Once you're outside, head straight ahead to the Abandoned Car Tunnels (there are a couple of Feral Ghouls in your way). Once inside the Abandoned Car Tunnels, look for the Brotherhood of Steel insignia and/or the words "To Mall Outpost" marked on the wall, and follow it. The same tunnels will also take you to Rivet City. Instead of getting out when you reach the Underworld "stop," you would continue heading south until you reach the end of the line.
and what does this have to do with chemistry