The structural integrity of the Pantheon's dome relies on an ingenious use of progressively lighter aggregates in its concrete mixture as it rises, culminating in lightweight volcanic rock at the oculus.

Roman engineers masterfully varied the composition of the concrete used in the Pantheon's dome. The concrete at the base of the dome incorporated heavy, dense aggregates like basalt and travertine, providing strong foundational support. As the dome ascended towards the oculus, lighter and less dense aggregates were progressively used, transitioning to pumice and tufa (porous volcanic rocks) at the uppermost sections. This gradient in material density, combined with sophisticated brick ribbing and coffers (recessed panels), significantly reduced the overall weight of the dome, minimizing stress and preventing collapse without the need for internal supports. This innovative technique allowed for the creation of an immense, self-supporting span.