Google Earth has added to its software a three-dimensional simulation that painstakingly reconstructs nearly 7,000 buildings of ancient Rome, including the Colosseum, the Forum, and the Circus Maximus. The program, which gives users access to maps and global satellite imagery, now hosts a new layer that allows surfers to see how Rome might have looked in A.D. 320, a bustling city of about 1 million people under Emperor Constantine. Pop-up windows provide information on the monuments, and visitors also can enter some of the most important sites, including the Senate and the Colosseum, to observe the architecture and marble decorations. Google Earth’s “Ancient Rome 3-D” is based on a simulation created by an international team led by the University of Virginia and the University of California. Using laser scans of today’s ruined monuments and advice from archaeologists, experts worked for about a decade to reconstruct ancient Rome within its 13-mile-long walls, said Bernard Frischer, who heads Virginia’s Institute for Advanced Technology in the Humanities. The simulation, which was completed in 2007, was intended as a scholarly tool to study the ancient buildings and run experiments on them—for example, to determine their crowd capacity. Frischer said the work’s publication on the internet means it can be used for broader educational purposes. Google has started a competition for U.S. teachers offering prizes for the best curriculum that uses the new tool.