Is there a underground mod like this?

[synthX139]

An elevator that leads to an underground part of a settlement you can place from the Build Menu.

[montky]

there are a few.

AnyDoorToAnyWhere, Placeable elevators,

basement living, the underboston etc.

 

 

 

a really simple way to accomplish it;

is to use the 'Cavern" / "Sunken Basement Egress" turned 90degrees,

and have the elevator cross under that threshold.

 

if you want to have elevators like Subway Cars or mine carts etc,

I recommend checking out Cypher2012's stuff.