Millions of tourists visit Angkor Wat in Cambodia every year to marvel at its remarkable architecture, yet most are probably unaware that when it was built nearly 1,000 years ago it was even more impressive. Using remote sensing technology, scientists now know what is hidden beneath the nearby paddy fields and jungle: a sophisticated metropolis with an elaborate network of houses, canals, boulevards and temples covering 30 square kilometres that housed three-quarters of a million people. To put that into perspective, London at that time was home to just 18,000. These previously hidden finds tell us a great deal about life during the golden age of the powerful Khmer dynasty.