Here, Yourcenar's Hadrian is being self-conciously modest about one of his most significant building projects, for the Pantheon (a temple to all the gods of ancient Rome) is among the most remarkable, influential and innovative buildings of the Roman Empire. The original Pantheon was built by Marcus Agrippa in c31 BC, but the building that stands today is Hadrian's, begun in c126 AD. The portico, with its monolithic columns of grey granite imported from Mons Claudianus in Egypt, leads into an enclosed circular space covered by the most extraordinary feature of the building, a concrete dome with a circular opening or "occulus." The dome itself remains, to this day, the largest unreinforced concrete dome ever built, and from it are ultimately derived all of the most significant domed buildings in the world, from Saint Peter's Basilica to Saint Paul's Cathedral to the circular reading room of the old British Library. Probably Yourcenar took the idea for this modesty from the fact that Hadrian allowed Agrippa's name, rather than his own, to appear on the pediment.