Still waiting for Cobra64’s explanation. But the walls are bolted to gether to prevent EM transmissions from getting out. Also look at edge of the inner door and door frame. That is a tight fit. Looks like more EM isolation. Inner floor looks raised and probably sealed below. What is inside? Console containing critical data that owners don’t want anyone to access unless they are supposed to? One can make educated guesses.
Of course, the other approach is to google “Astron LOFAR” and see what it is. (That is the name of the picture Cobra posted). Radio astronomy application in Holland.