In Multiplayer if you go to the level "Blood Gulch" you can park a tank with its back up to the wall of a base and then take a Ghost or Warthog and ramp over it. If you do it correctly you should land on top of the base.(Warthogs are harder to get up.)This works well to surprise you're enemy or to just be mean and stick the Warthogs and Ghosts inside so no one can get them out easily.
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