How to Obtain:
Obtained when you get your first document log – this could happen as early as in the text part of the game, or in the 2.5D section of it. In the text part, you first need to use the fishing pole twice on the pond – you will get a key the first time, and a severed hand the second time. Once you have the hand, use it, and have it scurry away.
While you wait for your hand to come back, go to the shed (without the light/lantern on) and use the surprise. You will get a 3 digit code etched on your arm. You can type look at the arm to look at it after if you forget. Once the hand returns to you, it will have a map to get you to the hidden place – go there, use the keypad, type in the code, use the box, and boom – you have the achievement.
If you’re already past that part, but still want to get it, you have to get to the 2.5D section of the game – when your game turns into your favourite colour. Once you’re at that point, you will occasionally get tugged at by your arm when there is an arm like/coloured grass poking out of the ground. Dig it out, and you’ll get the achievement.