In World of Warcraft, the Human capital, Stormwind City, is home to a large "underground" terrain that is often simply refered to as "below Stormwind" or "below SW."
There are countless methods to glitch your way under Stormwind. They all, however, involve wall jumping. Among the easier ways to access the are below stormwind is through the section of wall supporting the arch to the left of General Marcus Jonathan. Use the wall jumping procedure to climb up the wall. Near the top of the wall, to your right will be the roof of a section of fortification. When you feel like you can do it successfully, leap from your perch on the wall onto the surface and then head over to the far left corner of the platform. Wall jump the wall until you can jump onto the nearby ledge. Jump onto it. You will fall into the area below Stormwind. This will take several tries, (none of them are that easy) and there are easier methods, but this is the one I use.
Although, currently in Cataclysm after 4.2, the vast majority of these methods are no longer usable due to blizzard patching up all the holes in preparation for flying mount usage.
The shape of the area below Stormwind resembles a steppe next to a flat but barren plain. Scattered accross the place are large holes in the ground. While most of these will somehow teleport you back into a part of the city, one hole, the one below the far right edge of the wall separating the Valley of Heroes from the Trade District, is an abyss and if you jump down it, you won't die but you will be trapped, falling forever. Farther into Stormwind, the holes are absent, and it is entirely flat. The only mountains blocking your way are by the Dun Morogh region far north and other mountains around the edge. Other then that, it is just a vast plain where you can travel very far.