well when they opened it they found about 4 feet of water in the bunker. If you go to the site and look at the pictures it looks like water seeped in around the top.
It doesnt look like they sealed it very well 50 years ago. The concrete was supposed to be able to keep water out, treated with some chemical. but the lid probably should have overlaped the top and then came down around the edges and then sealed with foam or rubber to keep water out. It didnt help it was buried on the courthouse lawn and probably got 50 years worth of sprinkler water...