The pantheon is ancient Rome 's best preserved monument, situated in Piazza della Rotonda, a buzzing square in the heart of the city with a fountain and obelisk in its centre. Most historians agree that it was first built in 27 A.D. by Agrippa, as a rectilinear T-shaped structure. It burned down in the great fire of 80 A.D. and was rebuilt by the Emperor Domitian before burning down again in 110 A.D. when it was struck by lightning. Seven years later the pantheon we know today was built by the Emperor Hadrian who “wanted this sanctuary of all the Gods to represent the terrestrial globe and the celestial sphere, a globe in which the seeds of eternal fire are contained, all contained in the quarry sphere” (M. Yourcenar, Hadrian's memoirs). Greatly attracted to Hellenic culture, he built a temple incorporating the conservative yet decorative use of Greek classical orders, combined with concrete construction – a technique in which the Romans were especially innovative. Its unusual design consists of a rectangular porch linked to a huge dome with a large hole in the roof with a diameter of 9m. Sunlight coming through the roof ensures the unique lighting of the pantheon whose coffered ceiling also creates a special effect of light and shade and the rain forms a clear pool on the concave floor before leaving through the drain in its centre. No expense was spared with marble imported from all over the empire for use in the pantheon whose grey granite columns were quarried in Alexandria , dragged to the Nile, shipped to Ostia and brought up the Tiber.