Taken from the good fellows on gamefaqs. and Madjackal posting below.
Credit goes to Deluxey and Madjackal.
The list is now complete.
- organic world, go right from the starting point and feed the worm to pass.
- organic world, from the starting point, first intersection to the left. take the down-left path.
- organic world, leftmost area, you need to drill through the wall. when you're in, check the upper-left corner. there is an invisible path that you can use to enter the small room in the upper-left corner of the map.
- organic world, north from last boss room, requires missiles.
- organic world, dead end located NE of large circle (1st boss) room.
- organic world, pass through the poison cloud into the water. head left and drill up. check the map to locate.
- ocean world, first big water room, drill up on the right side of the waterfall and take the left path. there's also an artifact to the right.
- ocean world, same first big water room, medium height, at the right wall behind a spiral tentacle
- ocean world, again the same big water room, upper right corner, drill through the wall there
- ocean world, rapid water section, just enter from the left side like the first time, first intersection where you can decide directions (it has 2 grapple points), choose right.
- ocean world, just 1 room directly below #10. just drop down one intersection and choose right side.
- ocean world, big room east from the rapids, upper-left corner. missiles required.
- ice world, first huge cave in the north, ride the serpent all the way to the art pickup
- ice world, vertical room east from boss room. lower part.
- ice world, large room after avalanche area, ascend upper left corner, in the small room. it's shown on the map.
- magnet world, when you have to open the door in the ground, go in the purple pipe in the ceiling.
is in the magnet world, after you get the electrical power-up you come back here and its right before you start heading up to the pipe that brings you to the central part
Description #2 Thanks to Morris69:
The 17th is back through the Ice Room. Go through the tube as if you had just beaten the boss and you will be going towards another tube. At the end there is 1 room you couldn't have got in, and there is the 17th concept art.
- mechanical world, from the starting point go left and up, use electricity to power the gear and proceed to the piece of art.
- mechanical world, go to the clockwork puzzle (where you need to insert the colored gears), take the left path, connect the first plug, enter the gear, go south and enter the water, follow the water.
- mechanical world, same route as #19 but later, after the laser reflecting puzzle with movable platforms, follow the conveyor belt with the platforms all the way to the upper-left and then up.
- mechanical world, same area as #22 (twisting gears), go all the way to the left while staying below the giant drill, remove the cap from the nearest tube and enter.
- mechanical world, go to the gear puzzle, go straight up, when you get to the twisting gears section make your way through until you get to the giant drill, go all the way to the right while staying below the drill and remove the debris with your own drill, go down, enter the gear and follow the path.
is in the electrical world to the bottom left of the room where you have to destroy the 3 electrical things with the blue glowing thing you carry around the level if the that makes sense
is in the home world and you have to get before you leave the home planet, its located on the very bottom in a small room
My other helpful threads:
Map with important locations:
Easy final boss strategy:
Easy electrical boss strategy:
Crystal Puzzle Guide:
How to properly navigate the ocean current to reach the lower area:
Weird 100% bug and how to fix it.