Are you asking what it does? Or how to reach it?

The tile itself is called "The Underground" because it is a storage unit build on top of a large cliff-like cave system, which you can get into via the main entrance.

To find it you'll have to go to the north-east corner of the map. To open the main entrance you'll have to bring the "Code-Crate", which is located in another POI straight down the south. Just put the crate infront of the main entrance door in the slot and it'll open.

My answer was way longer and way more detailed than it should have been. But here we are.

Hope that answers your question. (You could've just written a comment but ok)

Originally posted by Pixelmaker:

My answer was way longer and way more detailed than it should have been. But here we are.

Someone should make a tutorial with lots of spoiler warnings.