Get Into Sea On Regret
From Halopedia, the Halo wiki
There is a way to leave the level Regret through the floor via a glitch. Go to the first underwater corridor. Kill the Grunts, and then go to the exit ramp. Walk to the side of the ramp, and crouch facing the 'banister' of the ramp. Then go up the ramp to the top, where the ramp changes from slanted to straight. Stand on the point where it changes to straight, but go a tiny bit to the side that is slanted. Then jump down and land just on the glass border, next to the stone floor. If you go through, sometimes you die, but if done correctly, you can get out into the area under the room (the sea), where there are AI controlled fish swimming. You are able to go into the lagoon where the big temple where Regret is, but you must use Aqua Chief glitch.