I've seen people on top of really high things when there's no flying vehicles. How do they get up there?
Here is a list of pretty much every single jumping technique in Halo 3.
The main one I've heard of and seen used is grenade jumping, where you throw a grenade at the ground and jump just before it detonates, boosting the player character as a result of the blast.
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I see all these beautiful halo videos on the internet, how do I take in-game videos and pictures?
Video:
This lists all sorts of various methods for recording 360 game footage. It would take a while to list them all; go through and figure out which one suits your situation best based on the equipment you have and the definition of your TV.
As for PC, you might want to look into buying FRAPS.
Screenshot:
On the PC, you can take a screenshot by pressing the print screen button on your keyboard.
On the Xbox, you might want to use a digital camera or look into using video capture hardware as detailed here.
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