We found the Ark! | Halo 3 

Lol, sorry for being late That "useless" room at 7:20 had something called a terminal. They give you bits of lore about the Halo rings and other canon events. There are others on different levels, but not all levels. W shield strat at 10:00 too 26:00 *t a n k* Autoturret at 32:20 is like a mini sentinel that is allied to you and helps clear our enemies. It stays in one place and floats. Those destroyed sentinels and stains are from a previous battle, i'm pretty sure. I cant find much info about it. It could be from a flood outbreak. New grenade type at 50:44... I wonder what they do!??!!? Invincibility does what it says on the tin... it makes you invincible for 20~ seconds