The Pantheon Oculus
The weight of the concrete dome is concentrated on a ring of voussoirs 9.1 metres (30 ft) in diameter that form the oculus, the main source of natural light in the Pantheon.
The thickness of the dome varies from 6.4 metres (21 ft) at the base of the dome to 1.2 metres (3.9 ft) around the oculus. The materials used in the concrete of the dome also varies. At its thickest point, the aggregate is travertine, then terracotta tiles, then at the very top, tufa and pumice, both porous light stones. At the very top, where the dome would be at its weakest and vulnerable to collapse, the oculus actually lightens the load.
