Waterfall Cave Level

This area is found in L Level. Bring a bird - or jump across the pit and enter the short door to collect a Grey Gun, then fire at the Yellow Bird to knock it into a position where you you can collect it.

Jump into the pit and float down - the Yellow Bird will slow your descent. Press button on the left. right, drift through middle, right, there is a pink laser (you can bypass by having a full Save Room queue, but avoiding the button press also works).

Find many gems, Cyan Beetle, Green Dog, Gunman Locator and a Heart.

Take the door back to L Level, way up high.

If you have an Orb, you can find a secret at the bottom here.