Vault Mega is the title.
A vault under ground but not just any vault, a gigantic vault. A underground type world with a 200ft tall ground (mud) ceiling. Vault mega is the main vault, (like a city in a state) and yes it's sealed up tight. Then there is a dirtland. (compare Dirtlands to the wasteland but Dirtlands is underground and its dirt every and dirt lakes with a very high ceiling meaning the Dirtlands are deep underground. where yes their is other mini vaults within the dirtland but vault mega is the biggest in the dirtlands.

I wrote more but this is the idea. I could see this being a DLC to Fallout 4 because it would be a Crazy adventure to be able to travel to the DirtLands to vault mega under the, wasteland and be able to switch from both areas freely.

Tell me what tall think馃憣

I think this goes into the Fallout General Discussion.

This

and not happening.