The large circular chamber in the center of the building on the ground floor is known as the crypt. In this somber space are forty Doric columns of brown stone, which contribute to the support of the flooring of the immense rotunda above. In the center is a marble star, which is theoretically the center of the city of Washington as originally laid out in the plan of L'Enfant, but, practically, far otherwise.

"Of course," writes Trumbull, recalling his proposed plans for the arrangement of this interior, " the staircase which I had recommended, together with the fire-proof rooms for the preservation of important records, etc, were sacrificed, and instead of the concentric walls and simple arches of my plan, to support the floor of the great room, a wilderness of truncated columns and groined arches were employed for that purpose, and this wilderness, called the crypt, very soon degenerated into a stand for a crowd of female dealers in apples, nuts, cakes, liquors, etc, for the accommodation of hackney coachmen, servants, negroes, etc, and becoming an intolerable nuisance, was ultimately denounced as such by Mr. John Randolph, and abated".