by the Romans, the Colosseum exhibits the arches and curves commonly associated with Roman buildings. The Pantheon in Rome still stands a monument and tomb, and the Baths of Diocletian and the Baths of Caracalla are remarkable for their state of preservation, the former still possessing intact domes. Such massive public buildings were copied in numerous provincial capitals and towns across the empire, and the general principles behind their design and construction are described by Vitruvius writing at the turn of millennium in his monumental work De architectura. The technology developed for the baths was especially impressive, especially the widespread use of the hypocaust for one of the first types of central heating developed anywhere. That invention was used not just in the large public buildings, but spread to domestic buildings such as the many villas which were built across the Empire. Materials The most common materials used were brick, stone or masonry, cement, concrete and marble. Brick came in many different shapes. Curved bricks were used to build columns, and triangular bricks were used to build walls. Marble was mainly a decorative material. Augustus once boasted that he had turned Rome from a city of bricks to a city of marble. The Romans had originally brought marble over from Greece, but later found their own quarries in northern Italy. Cement was made of hydrated lime (calcium oxide) mixed with sand and water. The Romans discovered that substituting or supplementing the sand with a pozzolanic additive, such as volcanic ash, would produce a very hard cement, known as hydraulic mortar or hydraulic cement. They used it widely in structures such as buildings, public baths and aqueducts, ensuring their survival into the modern era. Roman roads Roman roads were constructed to be immune to floods and other environmental hazards. Some roads built by the Romans are still in use today. There were several variations on a standard Roman road. Most of the higher quality roads were composed of five layers. The bottom layer, called pavimentum, was one inch thick and made of mortar. Above this were four strata of masonry. The layer directly above the pavimentum was called the statumen. It was one foot thick, and was made of stones bound together by cement or clay. Above that, there were the rudens, which were made of ten inches of rammed concrete. The next layer, the nucleus, was made of twelve to