It was sooo cool when Jim showed me. Previous owners had built a wall which blocked the bookcase from moving. It was impossible to see the rails built into the ceiling. When they were doing some remoldling, they were running some wires for an extra phone and got blocked by something. A little research reveiled the steel supports.
They actually called the police to come, they thought it might be some torture chamber, dead bodies, maybe a still. But, alas, like Al Capone's vault, nada.