The Colosseum is just one of many ancient buildings users can tour
Google has added a new twist to its popular 3D map tool, Google Earth, offering millions of users the chance to visit a virtual ancient Rome.
Google has reconstructed the sprawling city - inhabited by more than one million people as long ago as AD320.
Users can zoom around the map to visit the Forum of Julius Caesar, stand in the centre of the Colosseum or swoop over the Basilica.
Researchers behind the project say it adds to five centuries of knowledge.
"This is another step in creating a virtual time machine," said Bernard Frischer of the University of Virginia, which worked with Google on the Roman reconstruction.
"The project is a continuation of five centuries of research by scholars, architects and artists since the Renaissance, who have attempted to restore the ruins of the ancient city with words, maps and images," he said.
Also involved was Past Perfect Productions, which reconstructs archaeological and historical sites through virtual reality.
Joel Myers, the firm's chief executive, said: "Cultural heritage, although based in the past, lives in the present, as it forms our identity.
"It is therefore our responsibility to ensure its conservation, to nourish it and make it accessible, with the objective of promoting global understanding. Ancient Rome in 3D is a major step towards this goal," he added.
Ancient Rome is the first historical city to be added to Google Earth. Google's blog said the model contains more than 6,700 buildings, with more than 250 place marks linking to key sites in a variety of languages.
"Whether you are a student taking your first ancient history class, a historian who spends your life researching ancient civilisations, or just a history buff, access to this 3D model in Google Earth will help everyone learn more about ancient Rome," said Bruce Polderman, Google Earth 3D production manager.
Information bubbles pop up on more than 250 sites in the ancient city
Within ancient Rome there are some 200 buildings scholars know a lot about - classified as Class 1 -which Google says have been rendered as faithfully as possible.
The 3D models are based on a physical model of the city called the Plastico di Roma Antica.
The model was created by archaeologists and model-makers between 1933 to 1974 and housed in a special gallery in Rome's Museum of Roman Civilisation.
The new map was unveiled at an event in the Italian capital, and the modern day Mayor of Rome, Gianni Alemanno, praised the project.
"It's an incredible opportunity to share the stunning greatness of ancient Rome, a perfect example of how the new technologies can be ideal allies of our history, archaeology and cultural identity," Mr Alemanno said.
More than 400 million people have downloaded Google Earth since it was launched in June 2005.