The Baths of Caracalla are among Rome's most ambitious civic buildings using concrete and the vault to free interiors from the column and build large, uninterrupted volumes. Where the Pantheon admits the sky through a single dome, the baths solve the problem horizontally: roofing a series of vast interconnected halls without a forest of columns.

The solution is the groin (cross) vault. When two barrel vaults intersect at right angles, the load gathers not onto continuous walls but onto points at the four corners; the walls beneath the vault can therefore open up, spaces flow into one another, and windows rise high. The central cold hall (frigidarium) is roofed by this system: three giant groin vaults hold the concrete mass aloft while turning the floor into a columnless, airy volume. Roman concrete — mixed with pumice, cast into formwork — is the one material that makes these curved roofs possible.

But the building's real genius lies in its unseen parts. Beneath the floor and within the walls runs a system of voids called the hypocaust; hot air from furnaces below passes through these cavities to heat the floor and walls. Water is brought by aqueduct, stored in cisterns, heated, and distributed. Caracalla turns architecture into an infrastructure machine — not only to roof but to heat, to supply water, to channel crowds; it is the structural ancestor of the wide-spanning public interior that runs on to the basilica and the railway-station hall.