The Pantheon in rome has a giant 150' diameter, unsupported concrete dome. It uses some density "hacks" as well. Turns out the top third of the dome uses concre

The Pantheon in rome has a giant 150' diameter, unsupported concrete dome. It uses some density "hacks" as well. Turns out the top third of the dome uses concrete that's about 50% pumice (lightweight volcanic rock) by volume. Like ancient foam. Both structures are ancient at this point but the Panthon is particularly impressive in that the unsupported dome survives in a highly seismic area.