Crash Bandicoot 2: Cortex Strikes Back/Air Crash

A slightly harder version of ../Hang Eight/. You have to go up a stream partly by jumping on platforms and partly by riding the jet ski. This time of course, there are more dangers to avoid. Along the way you can easily find the crystal. Just after the first C box, there is a hidden portal that will take you to the secret Warp Room. Instead of getting on the jet ski, use the floating boxes on the right to reach a platform that is otherwise inaccessible. Once on the platform, you will be teleported to the secret Warp Room, where you can access the Snow Go level from a previously unreachable hidden location, which will allow you to reach the red gem.

The bonus area is located about two-thirds of the way through. Be careful not to fall while jumping on the boxes suspended in the air. Before the end you have to perform a long jump by jumping over an arrow box.

Getting the first gem
To break all the boxes in this level, you must first reach the level ../Bear Down/. Here in fact, there is a hidden portal that leads you to the secret Warp Room. From there, you can enter this level, but from a different location than where you started. You will have to pass a short area where you will encounter many dangers, breaking the boxes you find. At the bottom you will find the detonator to blow up the nitro boxes and a square platform. By stepping onto this platform you will be teleported directly to the conventional start of the level. From here, you just need to reach the end by breaking all the boxes to get the gem.

Getting the second gem
To get the second gem you have to get about three-quarters of the way through the level without losing any lives. If you succeed, you will find a platform marked with a skull shortly after where the crystal was. Climb onto it and you will be taken to an area similar to the rest of the level, short but very treacherous. At the bottom you'll come to an exit portal, with the gem waiting in front of it.