The Roman Pantheon

After the 2nd Triumverate and rise to power of Augustus, the
great General Marcus Agrippa turned his attention to civil
construction. The Appian Way and the Aquaducts of note, it is
perhaps the Pantheon to which all others pay homage. The
largest dome on earth for 100's of years, its concrete ceiling
was a single pour onto a wooden frame high above the floor.