I think the Tuatha de Danaan may have been nagas.

And they did move underground.

Now, there's underground and there's Hell.

They didn't go to Hell.

They went to live in underground worlds, cities. We know such cities exist.

If you've been to the DC area, then you know there are whole shopping centers underground -- one of them is Crystal City.

It's not a big thing. You get used to it.

https://en.wikipedia.org/wiki/Crystal_City,_Arlington,_Virginia

Crystal City is an urban neighborhood in the southeastern corner of Arlington County, Virginia, south of downtown Washington, D.C. Due to its extensive integration of office buildings and residential high-rise buildings using underground corridors, travel between stores, offices, and residences is possible without going above ground; thus, a large part of Crystal City is an underground city.

Now, here is something interesting:

Crystal City includes offices of numerous defense contractors, the United States Department of Labor, the United States Marshals Service, and many satellite offices for The Pentagon. It is also the location of Ronald Reagan Washington National Airport.