Underpass takes place in the rain, underneath the destroyed remains of a highway overpass. It's a great map for various types of weapons. It has narrow corners that make for unrivaled camping spots and wide curves that can hold devastating firefights. Players must be careful moving around, as it is a perfect map for hiding claymores and staging ambushes. In spectator mode, one can see the rest of the highway, and a large bridge.
Players must also be careful when using killstreaks, as the overpass can make using them difficult.
In a small building in the middle of the map, there is a blue poster on the wall with the word "Roycewicz" and a phone number. Roycewicz is the private the player saves inCall of Duty 4: Modern Warfare in the mission "The Bog", and is a lead member of the Infinity Ward map design team.
There is a poster up on many walls advertising a Infinity Ward "Rock Fest" underneath an African-American man with the name "Griggs". and has the Infinity Ward symbol on the top-right corner. This is a possible reference to Staff Sergeant Griggs from Call of Duty 4. Griggs is also the name of an Infinity Ward member.
There is a teddy bear in a warehouse on the catwalk.
There is a small banana plant on this map which can't be shot through even with FMJ equipped.
Some of the shipping containers have "IWCOD" written on their doors.
On the wooden walk on the bridge there is a take out box that has a smiley face and a message that says, "Have a nice day".
There is a mattress on the ground near the sniping spot at Flag A on the side with no ramp, and the player can jump from the higher floor onto it and not get damaged, much like the mattresses in Afghan.
Outside of the map is a hidden river with boats.
There are posters that say "Glacier" and depict the Titanic crashing.
In 2010, a video was released on YouTube where you could get a "secret old lady" to appear. After the release of the video, many theories were created as to whether you could actually get the old lady to appear. The video can be found here.