Find the level with the sandy beach, two treasure chests, and rock-nose formation. Open the two chests and drag the two crystals out of them. Put both of the crystals in each of the "holes" in the rock at the center of the map, like so:
The rock-nose will rise up and uncover a secret entrance gate, along with a large faucet that will drop a water gun. Use the water gun to spray the gate and it will open. Walk inside and you will be transported to a (secret) level, but not the final one.

Once everyone spawns in this level, go to the water guns on the right of the map. Use those water guns to spray the large wooden platform on the center of the map (where you spawn).

When you've done that, the platform should open up and uncover a boat with a large key inside it. The key acts like a weak melee weapon so be careful around your fellow players. Bring the key to the lock at the back of the map and it will unlock, allowing you to continue further.

This is the third and final level before the boss battle. The only thing you have to do here is pull a handle hanging from the roof at the left and right side of the screen. It's not necessary to pull both, just one of them. Use your Raise Arms key for easier grabbing.

Once you have pulled them, the submersible in the middle will open its glass hatch and everyone needs to enter it before the timer runs down.

Finally at the boss battle, you have to face the mutant octopus man from the trailer, or you can just call him Super Kevin.

We didn't get to see what happens when you beat him, because he wiped the floor with us. Any information is helpful and appreciated!

To get to the Space Race secret level you need to activate all three buttons on the Rocket launch map and grab onto it before it launches off. You can use a box if you don't have three people, just drag it onto the button and it should work.

Space Race Level: