From Tekkenpedia

Underground
Walls	Yes
BG Music	Mob

Underground is a stage in Tekken 4. The fight takes place in a room where many people cheer the fighters on. They are near or on some boxes, crates, and barrels. Those people and the objects count as the walls. A small bit of the floor is elevated on one side of the stage. That part of the floor can barely be stepped on.If a fighter is knocking their opponent into the walls, this can cause a cheering person to fall to the ground.