Architectural and architectonic experimentation with the self-weight of concrete dates back nearly two thousand years. One of the best known lightweight concrete structures is the Pantheon in Rome (ca. AD 128). The construction of its hemispherical dome, with an interior diameter of more than 43 metres, marks a climax in the progress of architecture and engineering. The Pantheon is a feat of structural engineering which was designed with great creativity and experience in terms of the details of the construction, building operations and building materials. The dome is made from a lightweight concrete mix which was obviously designed to reduce the selfweight of the structure. It was the advanced knowledge of the use of opus caementitium that encouraged the designers to plan this unique project and, indeed, to build it.