Visitors to Rome are often stunned when they see the Pantheon and learn it was built almost 19 centuries ago, during the reign of the emperor Hadrian. From the front, the building has a classical style echoed in neo-classical government buildings around the world but, as visitors walk inside, it is the amazing dome which causes them to gasp. At 43.3 meters (142 ft.) in diameter, it was the largest dome ever built in its time, and no larger dome has — in all the centuries since — been built in the same way. The dome of the Pantheon is a monolithic structure of concrete, whose beauty and antiquity attests to the versatility and durability of this building material which has become a ubiquitous part of the modern world.