Head back to the waterfall, near the water slide. There is on top of it, a crystal which you need to take down with an explosion. (Grenade, Ray gun, and so forth).
Once it's down, shrink it with the 31-79 JGb 215. Then, knife it and follow it until it sticks itself in the geyser leading to the spawn.
Activate it (by simply standing on it), to project the crystal on a tower, completing the step.