Underground is set in a London Underground station. It consists of two main areas, the station itself and some walkways around the tracks. The tracks are covered with demolished trains, which can be climbed on and walked through. The main spawn points are at either end of the map, one at the main road and the other at the station's 'Way Out'. The main station has glass roofs, so air support can reach there.
The players start on the north side of the map. The equipment armory is located there as well. The Weapons armory is located in south side of the central building, just beside the main stairs. The Air Support armory is located on the south most area.
There are several advertisements around the platforms, advertising game shows, local train routes and the Museum of History.
The trains out of the map have destination Westminster, and the trains inside the map have destination Middleton. Only one train breaks this rule. Under the bridge near the B Bomb, one train has its destination as Westminster.
The train destinations for Middleton are in fact just placed over the original Westminster destination.
There are newspaper stands in the station concourse that include a picture of Robert Bowling and the title "Man with beard invents time travel."
The map styled after the famous Tube map has multiple puns and references to the Call of Duty series and the companies involved in developing.
There is a wall clock on the map that says the time is 10:10, but the station clock time is 5:13:07, and also the first train is meant to leave at 20:25 although there are already trains docked in the station.
In the Multiplayer reveal trailer, the two factions fighting are Inner Circle and Delta Force, but this was eventually changed to the Spetsnaz and S.A.S.
If one flies under the map via spectator mode or noclip the player can see what appears to be an image of the River Thames.
There is a magazine that says "President plays Spec Ops. His State of the Union advises America to play it".
Beside the magazine stand next to the stairs leading into the central building of the map, there are two stands, each having a small sign on the very top of them that read "SHG Editions D'art". This is a reference to Sledgehammer Games, one of MW3's developers.
If the player enters the two double-decker buses via spectator mode, they can find magazines on the floors and seats reading "IW Mutant Buffalo Returns!"
In a stall in the station, there are two different colored packs of sweets (brown and yellow) that resemble M&M's but say "CofD's", a reference to Call of Duty.
When playing Infected, attempting to camp on the balcony and signpost beside the office will automatically kill the player. This was done to stop players from hiding there indefinitely.
Outside the station there is a parked taxi. The registration on the cab is LT09 XJP. However, if the taxi is blown up, the registration is IW52 LSA.