Yeah, there's two key fragments hidden in the world, hints are given below by others. Can't find one of the parts though.
From my bad memory, one of the pieces is in one of the temples (not the final one).
Basically a one room thing there and at the top, where waterfalls move left with black stuff in the way, there is a hole in the top of the room.
other one is, if I remember correctly, from the very bottom of the world, the path to the left (which also leads to a normal key piece).
there is a section where you fall down through some winding black goo paths, having to collect fuel and fly around, basiclly "dont touch the wall while flying stuff.
at the bottom of that, to the right is a wll with a tiny opening above it.
normal way to go here would be to go left and up through another goo tunnels flying section.
if you looking for it, you will easily find the opening.
Getting in there ia the hard part cause after all the goo dodging while moving downwards, you have to keep a certain amount of fuel.
and obviously react fast enough so you can fly into that opening.