OK this doesn't work all the time just sometimes. on the map blood gulch get in a warthog and drive to one of the caves and then get out of the warthog. throw a grenade in front of the cliff beside you then jump up on the cliff and you appear on a rock platform over the map.
this works sometimes but sometimes it doesnt. in the map blood gulch get in a warthog alone. drive to the cave, then get out of the warthog(as i said sometimes this doesnt work) and throw a grenade in front of you, and your just suddenly transfered up in the sky and then a rock formation platform appears under you and you land on it. if you have a sniper you can snipe your opponents off. the only way you can die on the platform is if it disappears and you fall and die or if your shot with a rocket launcher. this works half of the time.
At the beginning of the level: Assualt on the Control Room, when master chief goes off the pelican you can go on by pressing the button that has to enter a vehicle. You will eventually die in like 5 seconds.