The word arena comes from the Latin word for sand. The "arena" of the Colosseum was actually a wood floor that was covered with sand. Underneath, as you can see here, was a labrynth that held prisoners wiating to fight and had 2 tunnels. The one tunnel led to a reserve ~70 meters away where they kept the wild animals and the other tunnel was about 30 meters long and led directly to the gladiator school. The stage at the far end is being rebuilt to give theatrical performances on.