Stitches, did you even read the intro post? Within the post, its explained how to get into the city and where it is.

[indent]This specific Underground City is located under Omashu in the Earth Kingdom. Its entrances are built into the Omashu sewer system and secretly tunneled down deeper into the ground with several hidden entrances and exits. One can enter the Underground City by entering the Omashu sewers or via one of three cave entrances 2-3 miles outside Omashu that head downward until they reach the maze of tunnels made by the Earthbending engineers. The City is then ventilated with a series of large shafts built into the cliff, far below the city of Omashu, using a fire that heats air to rise so that cooler air from a different set of vents and constantly oxygenate the entire area while a) feeding the fire and b) providing a light source using several glass panes and chandelier-like mirrors, as to minimize the actual size of the fire needed thus reduce oxygen depletion.[/indent]