The cylindrical interior of the Pantheon is beyond magnificent! Above your head is the world’s largest unsupported dome weighing 5,000 tons. The concrete dome’s diameter is 142 feet and so is the distance from the marble floor to the oculus at the apex. The windowless hole in the ceiling measures 25.6 feet across. This is the only source of light in the Pantheon. The dome is decorated with five rows of 28 coffers (sunken panels).