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 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 a link below that explains it enough. <object width="425" height="355"><param name="movie" value="http://www.youtube.com/v/a1YDB8eDUKs&hl=en"></param><param name="wmode" value="transparent"></param><embed src="http://www.youtube.com/v/a1YDB8eDUKs&hl=en" type="application/x-shockwave-flash" wmode="transparent" width="425" height="355"></embed></object>
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, one in Outskirts (although this one is most likely a graphical glitch) 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.
Halo 3
There is also a newly discovered tri-gate in Halo 3, way outside the ark. It looks like many things to different people, such as an infection sign or master chiefs face. On the level the storm, there is a simple glitch to get out of the level, revealing three mysterious gates.
