IIRC there was originally a plan to have a big library or museum down there when the monument got built, but that got scrapped so they were left with an excessive basement space.
Or, you know, aliens.
These people should read up on well known bunkers like Project Greek Island at The Greenbrier Resort in West Virginia. A secret bunker was built at the resort during an otherwise routine renovation in the 1950s as a shelter for Congress. It was decommissioned in 1992 after The Washington Post revealed its existence.
I’m old enough to remember basement areas in places like high schools that had “fallout shelter” signs on them. This was the '80s and they weren’t used for that purpose, but in the 60s, they had this idea that people en masse could go to public fallout shelters and wait out the radiation. Unfortunately, they decided on rather random places (like the many-windowed stairwell at one of the buildings in my alma mater), so it wouldn’t have even work.
Also, we had one of these as a trash can in my high school auditorium. All of my high school friends (and my former drama teacher who I’m in touch with on social media) remembered it fondly when I found another one in an antique store and shared a photo of it.
They really thought everyone would just go down to the local schoolhouse basement, wait a week and everything would be back to normal.
The Post Office in the town I used to live in, only 10 miles or so from downtown Boston, still has its fallout shelter sign on it.
Ha yeah, Monument Mythos was a pretty good series.
