First monumental dome

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.

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