Mystery of the Gate
From Halopedia, the Halo wiki
The Mystery of the Gate Glitch requires the Sputnik skull enabled before starting the walkthrough (a Sniper Rifle would help too). Go to Delta Halo on any difficulty and progress until you get to the part where you must go down the sloping ramp into another level. Get close to the door, and Explosive jump to one of the ledges on your left or right, then grenade jump again to the top of the building. Make your way across to the opposite side, where you will have to grenade jump to the platform below (this is the spot where you would fight the Hunters). Go to where the Gondola would dock and zoom in and look slightly to the right; there will be a floating switch.
Any enemies that you encounter on the way, except the first two Jackal Snipers, will not be able to see you.
The High Charity Button
There seems to be another gate on High Charity. There is no way to reach it since it is floating some where in the room were it is located at.
There are also mysterious triangles, hidden in Halo 2, that are rumored to be connected to the mystery of the gate. There is one outside Delta Halo, one outside Cairo Station, one outside Terminal, several in and outside Outskirts (although these could be graphical glitches) and one in Metropolis. There are various videos on Youtube, some of which are linked below. They may be connected to the spawn points for the secret skulls. It is possible though unlikely that they are actual gates. They may just be graphical errors, or one of Bungie's favorite shape.
There has been a Youtube video claiming there is a tri-gate in the level Keyes; it showed the player by the Proto-Gravemind looking down the ramp, and it showed a black triangle on the floor. There was another one close to it right by one of the doors.
In Halo 2 there is a big triangle underwater in the level Delta Halo right under the second ruins on the level. On Metropolis there is a small triangle under the door that leads to the docks behind the building. On Cairo Station there is a triangle on a blue window, and this can be seen outside of the level. On Outskirts behind the Pelican you can see a black triangle. Right by the Blind Skull, if you look at the left corner, there will be another black triangle and some people say that if you throw a grenade by the triangle next to the skull, it will go out of the triangle by the Pelican. There is also a gate on The Great Journey at the end of the level where the Scarab breaks down the door. To the right of where it stops moving is an alcove where the gate is. It is unreachable in a banshee (unless you glitch) due to a barrier. If you stand on the Scarab's leg as it is moving, it might launch you into the alcove (or kill you).
There is also a newly discovered Tri-Gate in Halo 3, way outside The Ark. Many saw the shape as an Infection sign or a Prophet or Master Chief's face, due to the different perception of different players.