The Comet is a small white two-door hatchback. Walk along the highway until one appears, then jump on its roof. A prompt to do a "Vehicle Surfing" move will appear. Press [Action] to begin car surfing. Maintain your balance to remain on top of the car until it explodes to get the "Gnarly Wave" achievement. Although this can be done solo, it is easier to do in Co-Op mode. Find a comet that is not burning. Have the other player enter the Comet, then have your character jump on its roof and begin surfing. Have the other player find a lava pool, and drive the Comet into it, but be careful not to flip the car while doing it. A few seconds after entering the lava, the Comet will explode. Note: Do not attempt to destroy the Comet by crashing, shooting, or starting balls of fire.