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 , 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 square kilometres that housed three-quarters of a million people. To put that into perspective, London at that time was home to just ,. These previously hidden finds tell us a great deal about life during the golden age of the powerful Khmer dynasty.