In the Mountain levelPlay the level like normal until you get to the first train car.
Then go to the right of the map and along the cliff side
you will see a path of platforms
that you'll need to jump on. follow the path and it will lead you to a cave.
Go into the cave and grab the lantern to the left.
You're kind of on your own from here. You now need to find the 7 glowing green cubed gems. Finding the gems is the hardest part
because the cave is very confusing
so expect to get lost very easily. I offer no help finding the gems within the cave, but I can tell you there are ONLY 7 gems
. Once you have found all 7 put them close together in a pile
and you'll get the achievement.