Another innovative feature of the Colosseum was its cooling system, known as the velarium, which consisted of a canvas-covered, net-like structure made of ropes, with a hole in the center. This roof covered two-thirds of the arena, and sloped down towards the center to catch the wind and provide a breeze for the audience. Sailors, standing on special platforms, manipulated the ropes on command. The Colosseum incorporated a number of vomitoria — passageways that open into a tier of seats from below or behind. Each vomitorium was designed so that the immense venue could fill in 15 minutes, and be evacuated in as little as 5 minutes. Each entrance and exit was numbered, as was each staircase