The dome is 20 feet thick at the base and 7.5 feet thick at the oculus.

The concrete dome is 20 feet (6 m) thick at the base and tapers to 7.5 feet (2.3 m) thick at the oculus. Without materials like rebar to hold the dome in tension, the massive rings of concrete were used to create a buttress-like effect that forces the dome into constant compression. The concrete dome weighs an astounding 5,000 tons (4,535 t).

Anthony Majanlahti / Flickr / CC BY 2.0