Mystery of the Gate
From Halopedia, the Halo wiki
300px|right 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 Grenade 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 is a link below that explains it enough. http://www.youtube.com/v/a1YDB8eDUKs&hl=en"
Tri-Gates
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.com, 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.
Halo:Combat Evolved
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 and there was another one close to it right by one of the doors.
Halo 2
In Halo 2 there is a big Triangle underwater in the level Delta Halo right under the second ruins on the level, and on level Metropolis there is a small triangle under the door that leads to the docks behind the building. In Level Cairo Station there is a triangle on a blue window, and this can be seen outside of the level. On Outskirts by the behind the Pelican you can see a black triangle, and 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 hitting another player if you play with two players.
Halo 3
There is also a newly discovered Tri-Gate in Halo 3, way outside the Ark. Many saw the shape as Infection sign, Prophet or Master Chief's face, due to the different perception of different players.
On the Campaign level The Storm, there is a simple glitch to get out of the level, revealing three mysterious gates.
Another Tri-Gate in Halo 3 would be the one on the The Covenant. It is on top of the third tower and the only way to get to it is by getting out of the map with a Hornet or Banshee.
