Fallout 4: Far Harbor is a large DLC and of course with the majority of these larger expansions we get another look at a vault. This is of course Vault 118 which features the tragic story of an experiment of the rich with the poor that did not go according to plan.
I won't go forth on any spoilers as I'm sure you're eager to explore the location yourself. Vault 118 is found in the North East quadrant of the Far Harbor map at the "Cliff's Edge Hotel".
You can be guided there by a robot at Far Harbor via a quest or just explore it yourself like I did. When you're at the hotel you can see the vault entrance above though you need to actually sneak into the underground area of the hotel to access the vault.
Once you reach the interior of the hotel you need to head to the far right and climb up to the top of the hotel. Then follow it the other way around and basically just the follow the path until you reach the basement with a jukebox and two helpful turrets. The elevator to the vault is at the far back beside the turrets and you just go down from there.
Be sure to mention you're the detective and you gain entrance along with a fun side quest. Alternatively you might just be able to use an elevator to the left when first entering the building as I was able to ride it up there, it's worth a shot. It should mostly be straight forward and if you have any questions let me know.
Read Our Fallout 4: Far Harbor Review
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