The Infinite Marines Glitch can be done on the Halo 3 level, The Storm. After defeating the Scarab and going through the aid station you will come across several Marines fighting some Brutes in a large storage room. Defeat them and go into the next room and you will find several Voi Factory Workers fighting the Brutes. When you enter this room let all the Marines and Voi Factory Workers get killed. After they are all dead, 5 Marines will spawn in the previous storage room and join you. If they die then more will keep spawning indefinetly. You might need to walk deeper into the room, near the Brute holding the worker, in order for the Marines to join the fight or they won't follow. For some entertainment you can lead the Hunters in the next area back to this storage room and watch them destroy countless Marines or you can start betraying them. You can turn on the Tilt and Mythic Skull, the Brutes and Hunters will be nearly invincible. The glitch can be very useful on Legendary because infinite Marines will eventually kill off the Brutes and Hunters. They stop spawning when you go out outside.
Also on The Storm when your fighting the Scarab, the Marines firing the Missle Pods on the catwalk leading to the aid station will continuelly spawn if killed.
Infinite Marines can also be found on The Covenant. When you get the Hornet and are flying to the third tower you will see an island with an Anti-Air Wraith on it. Destroy the Wraith, land on the island, and drop your Hornet into the water. Another Hornet will come and land, then drop it into the water and another will show up. Each Hornet carries 1 Marine and will keep spawning, although the map will overload eventually.
Co-Operative Fun At this part is a good time if you are playing co-operative to start killing marines. It provides an endless challenge because of the infinite marines. however, one player must stay back and snipe because the others who attack directly will die eventually and need to spawn