The Vanishing of Ethan Carter

The Curse of the Sea-Thing is a puzzle located in the mines directly after netting the Anger achievement and just before you leave the mines through the bubbling water.

As you make your way out of the mines you'll see a path on the left that leads down many steps to a doorway. Read the note on the door and then enter. You'll be inside a maze and will need to locate and inspect 4 bodies to open a gate to the puzzle. Be warned though, there is a dangerous miner inside the maze who will charge the player if you get to close. If you encounter him it's best to back away and wait for him to change direction. If he gets you, you will be teleported back to the start of the maze.

3 of the bodies are on the outside of the maze with the final corpse at the foot of a hill that leads up to the puzzle itself. After locating the third body simply follow the orb to the hill and the last body. Ascend the hill where you will find a plate that must be sensed to activate the puzzle. You'll need to rotate symbols in the correct order to trigger a flood and witness tentacles whereupon you'll find yourself underwater. From here, turn right and head towards the wheel to find the note and unlock the achievement.

The video below shows how to find all corpses and complete the puzzle.