From the book:

Twelve years had passed since I had laid the body of my great-uncle, Captain John Carter, of Virginia, away from the sight of men in that strange mausoleum in the old cemetery at Richmond. Often had I pondered on the odd instructions he had left me governing the construction of his mighty tomb, and especially those parts which directed that he be laid in an OPEN casket and that the ponderous mechanism which controlled the bolts of the vault's huge door be accessible ONLY FROM THE INSIDE.