Google Earth has launched a 3D reconstruction of ancient Rome as it may have been in 320 AD.

The virtual traveler can now see every building as it stood in Rome at that time. In reality, just 300 buildings of classical Rome have survived, in most cases in ruins.

The 3D model is visible on the website as a layer. The layer floats a few metres above the satellite image of present-day Rome so that users can have a sense of locating where the ancient structures once stood.

According to the Google Earth blog, the 3D models are actually based on a physical model of the city called the "Plastico di Roma Antica" - created by archaeologists and model-makers from 1933 to 1974 and housed in a special gallery in the Museum of Roman Civilization in Rome.

An extra feature in the 3D city is that 11 buildings have viewable interiors. These include: Basilica of Maxentius, Colosseum, Forum of Julius Caesar, Ludus Magnus, Temple of Venus and Rome, Temple of Vesta, Regia, Basilica Iulia, Basilica Aemelia, Curia Iulia, Tabularium.

The Guardian reports: "They (users) can float through the Forum, past the platform or "rostra" from which Cicero once declaimed, admire the statues, read the inscriptions, pry into palaces, and then slip round to the Colosseum or whisk over to the Circus Maximus where the ancient Romans held their chariot races."

Google Earth's chief technologist, Michael T. Jones, said, "It is the "Rome of [the emperor] Constantine in which everything is new. It's new. It's modern. It's beautiful."

The first concerted effort to "recreate" ancient Rome was made by an Italian architect, Italo Gismondi. Three years before his death in 1974, he finished a vast, plaster model of ancient Rome in 1:250 scale that can be seen in the city's Museo della Civilta Romana, the museum of Roman civilisation.

Gismondi's research played an important role in the digital project, which was begun in 1997 by a teacher at the University of Virginia, Bernard Frischer. After 10 years of work and collaboration with the Milan Polytechnic, "Rome Reborn" - made up of 50 metres of polygons (the building blocks of three-dimensional computer graphics) - was unveiled last year.

The job of transferring it to the web was shared between Google's 3D unit and a Rome-based firm, Past Perfect Productions, run by a Briton, Joel Myers.

Asked why 320 AD was chosen as the period for reconstruction, he said: "Because it was Rome at its moment of greatest splendor as far as its architecture is concerned. If you went back to periods of more historical interest, like Julius Caesar's, you would not have the Colosseum, for example."