The Basement Living mod is perfect for those that want to build storage spaces or shelters underground.
Instead of trying to decribe it ourselves, we'll let the developer handle that for us. If you prefer reading about the mod:
"Basement Living adds 10 completely standalone basements & bunkers (interior cells) each with their own standalone workshops and craftable door hatches, allowing you to place/enter into them from any settlement you choose. They work just like Home Plate, so you can tear-down and decorate them however you like! This was done carefully, and methodically, and does not alter ANY existing settlements or interiors whatsoever, nor does it require any batch files, god mode, or other workarounds. They're mostly copies of vanilla cells, so you may recognize a few. I found a couple cool unused cells and copied those too, adding a few things and lighting touches."
If you prefer a video:
It's pretty involved but it adds a nice little touch to any settlement.
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