The place to find this secret is in the second room of Sector 6

After you pass the first room, and go down the platform, you should end up in this room:



To get this achivement, you DO NOT need to solve the puzzle here. Over to the right of the room, you should see some broken cubes stacked on top of each other, as shown:



What you need to do is climb up on those cubes, and from there, jump onto the ones sticking out from the wall:



From there, make your way over to the incline (There's a little invisible wall that you have to jump over as well, don't think you can't climb up it):



Once there, just climb up until you find a hole:



Go into the hole, and you'll receive the achievement! I didn't picture it here, so you can see it for yourself!