Swim outside the game world:

Encounter the Boss in the "Blades..." level. It is a 
large room with a Boss that resembles a large 
dragonfly larvae. Go to the bottom of the room where 
you can refill you air supply with the bubbles coming
out of the ground. Next to the bubbles on the floor 
near the wall is a small hole. Go through it and get 
outside the game world.


by unknown