rain and jungle vegetation, asked NASA to make such a map in an inspired gamble that it would reveal interesting information. It was far from NASA'S priority of predicting weather systems and so on, but after much begging by Mr Stubbs and a seemingly miraculous delay in returning the shuttle to earth, the time was found. Tony Freeman and Scott Hensley of NASA'S Jet Propulsion Laboratory (JPL), Pasadena, became involved. Several new missions have followed since. JPL continues to interpret the data, which is no mean feat since, according to Dr Hensley, the radar images in their purest state 'look like fuzz on a television set'.

Radar is helpful because of the difficult landscape outside the main temples.

Forest in the north, rice fields in the south, and landmines from the civil war era, make it difficult not only to hike, but also to gauge the local geography, such as the difference between a mound and a general slope. Aerial photography', meanwhile, is of limited use: the forest canopy obscures the land; and such two-dimensional images cannot give a sense of height.

Radar, effectively a camera that uses radio waves for its flash, is different. The signal can penetrate through vegetation when necessary. It can detect moisture in its 'polarimetric' mode, because water changes the reflection of the signal. Most importantly, radar can detect height when several antennae send signals which interact with each other, a mode known as 'interferometric'. 'It is amazingly accurate', says Professor Roland Fletcher of the University of Sydney. 'You can tell how developed crops are by differences in their height and shape.'

Such topographical information is vital

because the Angkoreans were great 'landscapists'. They surrounded their shrines with moats, or built them on mounds. Moreover, they tended to live near dykes designed to irrigate rice farms. Radar has revealed many small ruined shrines outside the central area where the main temples are located. French archaeologists in the 1950s had noticed the remains of two temples at Kapilapura, north west of the Angkor Wat. Tony Freeman at JPL, however, noticed a mound--invisible to a hiker--that seemed suspiciously regular (Fig. 2). Elizabeth Moore duly inspected the site. She found the remains of six temples, not two, one of which is Fig. 3. Meanwhile, 30 km north west of Angkor, in an area at the time inaccessible due to Khmer Rouge occupation, a new temple that is still standing, Sman Teng, was discovered thanks to its 15 metre height contrasting with the surrounding trees.

The north of Angkor has yielded several small shrines. Since the shrines themselves have collapsed, they are 'flat' and cannot be seen on the radar. However, the radar map detects square shapes, which correspond to the square moats around monuments. The moats have since filled in, but the interferometric radar is so subtle that it detects the change in the height of the former banks. The site is then visited precisely with a Global Positioning System locator, and the shrine can be found, as was that in Fig. 4. It would be almost impossible without radar. 'The vegetation is so thick that even when you know it is there, the shrine can be difficult to see', says Professor Fletcher.

More significant than the detection of small shrines, however, is the recent discovery of innumerable dykes and lakes that reveal Angkor to have been an enormous sprawling city spanning 1000 square kilometres and probably containing as many as 750,000 people. An airborne (rather than space) radar mission in late 2000 covered the vast area, and information has been filtering out in the past two years.

The large temples are what we would call a "Central Business District",

akin to the City of London', explains Professor Fletcher, who runs from Sydney, with the Ecole d'Extreme Orient in France and the staff of APSARA, Cambodia's onsite management team at Angkor, the aptly named Greater Angkor Project. Sprawling and sparsely populated, as opposed to small and densely packed like Imperial Rome, Angkor has changed how archaeologists think of pre-industrial cities.

 This is an image of the area around the city of Angkor, Cambodia. The city houses an ancient complex of more than 60 temples dating back to the 9th century. The principal complex, Angkor Wat, is the bright square just left of the center of the image. It is surrounded by a reservoir that appears in this image as a thick black line. The larger bright square above Angkor Wat is another temple complex called Angkor Thom. Archeologists studying this image believe the blue-purple area slightly north of Angkor Thom may be previously undiscovered structures. In the lower right is a bright rectangle surrounded by a dark reservoir, which houses the temple complex Chau Srei Vibol. In its heyday, Angkor had a population of 1 million residents and was the spiritual center for the Khmer people until it was abandoned in the 15th century. The image was acquired by the Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) on the 15th orbit of the space shuttle Endeavour on September 30, 1994. The image shows an area approximately 55 kilometers by 85 kilometers (34 miles by 53 miles) that is centered at 13.43 degrees north latitude and 103.9 degrees east longitude. The colors in this image were obtained using the following radar channels: red represents the L-band (horizontally transmitted and received); green represents the L-band (horizontally transmitted and vertically received); blue represents the C-band (horizontally transmitted and vertically received). The body of water in the south-southwest corner is Tonle Sap, Cambodia's great central lake. The urban area at the lower left of the image is the present-day town of Siem Reap. The adjoining lines are both modern and ancient roads and the remains of Angkor's vast canal system that was used for both irrigation and transportation. The large black rectangles are ancient reservoirs. Today the Angkor complex is hidden beneath a dense rainforest canopy, making it difficult for researchers on the ground to study the ancient city. The SIR-C/X-SAR data are being used by archaeologists at the World Monuments Fund and the Royal Angkor Foundation to understand how the city grew, flourished and later fell into disuse over an 800-year period. The data are also being used to help reconstruct the vast system of hydrological works, canals and reservoirs, which have gone out of use over time. Research teams from more than 11 countries will be using this data to study the Angkor complex. P-45156