How do I jump onto really high stuff in Halo?

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.

Grenade or rocket jumping is how they do it. You have to be careful and willing to take some damage depending on which version of the game you play. It isn't very hard to do, but you do have to take some time to learn the timing. The best way to learn it is to run a local game, load a multiplayer map and load up on rockets and grenades. Then pick a place that has a high wall and time droping grenades onto the ground just in front of the wall. Time the explosion and then work on jumping above the grenade just before it explodes. You want to be just reaching the peak of your jump when the grenade goes off to boost you even higher. You can do the same with rockets, but it has to be a bit more precise. Practice a bit and you'll get it.