The underground pipes are, as their name would imply, pipes that are located underground and/or underwater. They can only be accessed when the player is transformed into the Yellow Drill.
In gameplay, when going into an underground pipe, the playable character will immediately be sucked into the underground pipe and taken far distances in an instance, allowing the player to take shortcuts through the levels and even enter sections or areas that would otherwise be inaccessible at the given point.
The underground pipes made their first appearance in both the Wii version and Nintendo DS version of Sonic Colors, and later the remaster Sonic Colors: Ultimate. In these game, they resemble regular pipes, though their appearance differs between the versions; in the Wii version and Sonic Colors: Ultimate, the underground pipes are green with red rims and white arrows at the ends, while in the Nintendo DS version they look like simple brown pipes with streaks of green.