This glitch requires the Keys To The City expansion, and can easily be done in Co-op mode. Go onto the pier that extends out into the ocean and spawn two ramp trucks back to back, with enough room to spawn a buggy in-between. Have a friend get into a ramp truck. Park the buggy on the ramp truck that your friend is not in. Then, have your friend back up into the other ramp truck (the one you are on). You should all go flying up into the air, with the possible exception of the ramp truck that the buggy was on. Note: While on the ramp truck, its recommended that you hold the hand brake (A) to keep from sliding off.