Indeed, the hole in the roof is 7.8 metres diameter.
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The Pantheon remains the largest dome constructed by concrete that is not reinforced.
Built in AD126, and was the basis for many similar domes over the following centuries.
Hey, what have the Romans ever done for us?
I remember myself and Mrs Tosh looking round the building some 14 years ago. I was absolutely mesmerised by the dome, and the fact that the Romans built it, not far off, two thousand years ago.