An abbreviation for Round Concrete Pipe. Common terrain when involved in urban exploration.
These pipes are the main travelways for both sanitary sewage and storm water run-off. During the majority of UE expeditions, one will try to avoid the sanitary sewers (for health reasons) and stick to the storm sewers.
As these pipes constantly have water flowing through them, they tend to get slippery, so great caution is advised while traversing the underground. If you have hard water in your region, time will produce some beautiful stalactite formations, or even a rippling effect on the floors of the pipe. Water seeping through the joints of these concrete pipes can also give the appearance of bleeding walls.