The Roman Pantheon is the boldest single gesture of ancient engineering: a single dome of unreinforced concrete, 43.3 metres across. That span went unsurpassed for more than 1,700 years; the height of the interior is exactly equal to its diameter — the dome is the upper half of an imaginary sphere that would fit perfectly inside.

What makes the dome possible is the systematic lightening of weight toward the top. The aggregate in the concrete mix changes from base to crown: heavy travertine and brick fragments below, lighter tufa higher up, porous pumice at the very top. The dome is about 6 metres thick at its base and thins toward the crown. The stepped coffers (hollow recesses) on the inner surface both reduce material and gather the load into ribs. The lateral thrust is met by the 6-metre-thick cylindrical wall carrying the dome and by relieving arches hidden within it.

The 9-metre open oculus at the crown is structural too: by removing all material at the dome's most stressed point it acts like a compression ring, and it is the room's only light source — a disc of light that travels across the walls through the day. The Pantheon thus unites concrete, geometry, and light in a single flawless volume; it is the building in which dome architecture is born.