The massive
Pantheon, built in 126 AD on the site of the original Pantheon that was destroyed in a huge fire in 80 AD. It was dedicated to all the gods of ancient Rome. The Pantheon is the best-preserved building of its time, and two thousand years after it was built the Pantheon's dome is still the world's largest unreinforced concrete dome. The height of the dome and the diameter of the interior circle are the same - 43.3 metres, so the whole interior would fit exactly within a cube (alternatively, the interior could house a sphere 43.3 metres in diameter - fun fact for the day :)