Robot World Arena is the first secret level in Marathon Infinity and is called a Co-Op Carnage Break level, as it's best reached in Co-Op. It also requires Co-Op to finish.
It's a very tiny level directly inspired by Waldo World Arena from Marathon 1. It's meant to train the Pfhor Fighters and conditioned ranks like slaves and the player, of course. it has a lot of valuable ammo to collect, including Rocket Launchers. There are only a few fighters on this level. The exit is only reachable with a second player helping you. He must activate the switch in the southwest room of the arena for you to get through the formerly locked door in the southeast room.
You can use the Uplink Chips in future levels.