breathtaking vista of plain and mountain and with axial symmetry, the sea, framed by the foothills of the apennines. here, roman architects began to play with the new curvilinear forms, such as this graceful hemicycle with its colonnade supporting an annular barrel vault inset with coffers that were once richly decorated. this amazing architecture was made possible by something which to us in the 20th century seems so commonplace it would hardly deserve comment-- concrete. concrete had been used before this time-- a simple mixture of three parts sand, one part lime, broken stone, and water. but among its problems for builders was that it dried very swiftly so it could only really be usefor layering courses never for building entire structures. but in the first century b.c., the romans discovered the almost magical properties of a reddish volcanic sand called pozzuolana, which comes from pozzuoli near naples. concrete made with pozzuolana has a very different quality. it's very strong. it can be used in wet conditions. it dries slowly, so entire structures could be built bonded from top to bottom as