“You thought it was as bad as it would get, and then it got worse.”

Alice M. Greenwald, the director of the National September 11 Memorial Museum at the World Trade Center, was recalling a feeling common among Americans on Sept. 11, 2001.

But she was also describing how the galleries depicting that day’s events will unfold for visitors.

Where much of the underground space is astonishingly vast and serene, the historical exhibition, contained within the volume once occupied by the north tower, is cramped and irregular. Deliberately labyrinthine, it is meant to jar those who see it.

Rounding one corner, a visitor will suddenly come upon the rear end of Engine 21 (“Keep back 200 feet,” it still commands), looking merely like an old fire truck that has seen a lot of action. A few steps more into the gallery, however, and it is revealed as a mechanical carcass.

The cab, all its trim and livery burned away, resembles a skull.

Transitions like this — by turns shocking and calming, distressing and heartening, awe-inspiring and grief-inducing — compose the memorial museum.