Its a Salt Mine in Romania

Ya it is in the United States either in upstate Michigan or Ohio.

haha thats pretty good. but we already know it's in the US

It would be a bit weird if it was in Romania. Aperture screams cold war era misplaced american money to me.

If you look at the display case at the entrance to 1940s/1950s Aperture, the newspaper says that Cave Johnson bought a mine in Michigan for his science facility.

Probably that place or something like it was the visual inspiration given how many sources Vavle said they used for the game. Cool place all the same.

^ those 2 comments
Valve use lots of stuff from Europe which inspired them.  They did it in other games(City 17 in HL), probably did it here.

" If you look at the display case at the entrance to 1940s/1950s Aperture, the newspaper says that Cave Johnson bought a mine in Michigan for his science facility. "