What makes you think the Hall of Records is below ground? In a sense it is buried but not below ground. Buried by many things other than earth and maybe a great deal of ignorance, but definitely not underground. Excluding the one in Atlantis Cayce said there were two halls one in Egypt and one in the Yucatan, he is correct the one in Egypt is underneath the Sphinx’s paw.
One thing you can be sure of is the guys who orchestrated this assembly of information were nothing short of pure genius to hide it in plain sight for so long.