Mystery of the Gate

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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 Truth
The gate is most likely the switch to activate the gondola glitched so that it appears early. To explain this it would mean Bungie had placed it there to only appear when the gondola is there and disappear after it is activated, through the skulls and glitching the game it appears. An odd thing, though, is that if you go through Regret and get to the building to the right of the gondola using Sputnik and then use a passage in that building to get on the gondola, there is no switch to be seen.

The High Charity button
There seems to be another gate on High Charity. There is a link below that explains it enough.   

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.

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.