Here's what you need to do (note the unlock criteria is subject to change from time to time). :)
- City Stage 2: Complete City Stage 1
- City Stage 3: Complete City Stage 2
- North Pole: Complete CIty: Stage 1 in The micro in under 1:24
- Desert Stage 1: Complete City: Stage 3
- Desert Stage 2: Complete Desert: Stage 1
- Desert Stage 3: Complete Desert: Stage 2
- Desert Stage 4: Complete Desert: Stage 3
- Snow Stage 1: Own the APC, The Big Rig, or the Tank
- Snow Stage 2: Complete Snow Stage 1
- Snow Stage 3: Complete Snow Stage 2
- Snow Stage 4: Complete Snow Stage 3
- City: Mayhem 1: Complete Desert Stage 1 in under 1:05
- City: Marathon: Travel 370m on City: Mayhem 1
- Desert: Mayhem 1: Complete Snow Stage 1 in under 1:00
- Desert Marathon: Complete Desert: Mayhem 1
- Snow Mayhem 1: Complete Snow Stage 4
- Snow Marathon: Complete Snow Mayhem 1
- Snow Mayhem Marathon 1: Complete Snow Marathon
- City Pro Racers 1: Complete City: Stage 2 in The Classic in under 1:00
- Desert Pro Racers 1: Complete Desert: STage 2 in The Buggy in under 1:00
- Snow: Pro racers 1: Complete Snow Stage 2 in The APc in under 1:18
- Pro Racers Marathon: Complete Snow: Pro Racers 1
- City Big Air: Own the Lowrider
- Desert Big Air: Complete City: Big Air in The Lowrider in under 1:03
- Snow Big Air: Complete Desert: Big Air in The Bouncer in under 0.55
- Big Air Marathon: Complete Snow Big Air
- City Rush: Complete Big Air Marathon
- Desert Rush: Complete City Rush in The Classic in under 0:45
- Snow Rush: Complete Desert: Rush in The Trophy Truck in under 0:50
- Rush Marathon: COmplete Snow: Rush in The Trophy Truck in under 0:39
- Volcano: Stage 1: Complete Pro Racers Marathon in The Racer in under 2:50 OR own The Beast
- Volcano: Stage 2: Complete Volcano: Stage 1 in The Buggy or The Beast in under 0:40
- Volcano: Stage 3: Complete Volcano: Stage 2 in The Monster or The Beast in under 0:58
- Volcano: Marathon: Complete Volcano: Stage 3 in The Bouncer or The Beast in under 1:10