Baia - The Temple of Mercury 1st c. BCE

"The world’s first ever monumental stone dome is the 21.55-m diameter Temple of Mercury at Baiae in Ancient Rome, near Naples, Italy. Probably constructed in the first century BC from packed tufa rubble, it was not, in fact, a temple, but instead a city baths, along with two similarly sized concrete domes. The dome is unusual for being pierced by both an upper oculus and four windows, and helped to inspire later Roman domes such as the Pantheon. It is not quite circular in plan, and probably used eight timber trusses during construction."

-taken from guinnessworldrecords link below

Illustration of the Temple of Mercury by Giovanni Battista 1765.

Portion of temple that is above water.

Source:

https://www.guinnessworldrecords.com/world-records/400869-first-monumental-dome/

https://archaeology-travel.com/france/puy-de-dome-mercury-temple/

http://www.stilus.nl/oudheid/wdo/geo/B/PLAATJES/BAMERCUR.html

https://www.atlasobscura.com/places/the-sunken-city-of-baia-baia-italy

https://www.architecture.com/image-library/ribapix/image-information/poster/imperial-villa-known-as-the-terme-di-baia-baia-tempio-di-mercurio/posterid/RIBA16970.html?Action=Cookie