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BACK in 2015, I stood in a grassy field in southern Illinois, looking at a map of the 1000-year-old neighbourhood buried right beneath my feet. Nearby, cars drove by on a narrow road and a group of squat fuel tanks cast long shadows in the early morning sun. The place looked like the outskirts of a typical farm town, but the two archaeologists with me had uncovered something that couldn’t be seen with the human eye: not far below the ground were some shadowy blobs, arranged in a circular pattern.

Sarah Baires is an archaeologist at Eastern Connecticut State University and her colleague Melissa Baltus is at the University of Toledo in Ohio. They study the Mississippian civilisation, a group of culturally linked Native American settlements along the Mississippi river that existed until around 1600. This field, which now seems like the middle of nowhere, was once a dense residential area of that society’s greatest city, known today as Cahokia.

Our only map of this ancient place came from a group of graduate students, who spent weeks trudging across the field with magnetometers strapped to their chests. These devices measure minute differences in Earth’s magnetic field, and they are excellent at picking up spots underground where rocks have been moved or there has been a fire. Places where people have burned wood or dug out the foundations of a house have slightly different magnetic signals than the undisturbed landscape, and those perturbations show up as dark patches.

At this site, Baires and Baltus found spots that were suspiciously symmetrical – mostly rectangles. When they realised these were arranged in a tidy circle, that sealed the deal: those blobs on the map were once houses, arranged around a circular courtyard.

Now Baires and Baltus knew where to start digging. After several field seasons, they excavated two buildings and a ceremonial feature called a borrow pit, a deep trough lined with brightly coloured clay. Researchers at Pompeii in Italy use a similar strategy: they mount ground-penetrating radar devices on wheeled carts, driving them around the areas of the city that are still buried under ash, seeking structures. That way, they don’t flail around wildly with picks and drills, looking for buildings.

Some of the greatest archaeological finds of the past two decades were made without ever lifting a shovel. Angkor is an abandoned metropolis, formerly at the heart of the millennium-old Khmer Empire in what is now Cambodia, but much of that site has been overtaken by jungle. So Damian Evans, an archaeologist with the French Institute of Asian Studies, worked with a team to mount a lidar device on a helicopter and fly over the ancient street grid of the city.

Lidar measures small differences in ground elevation, and Evans’s survey revealed that Angkor’s neighbourhoods once stretched far and wide, housing nearly a million people outside the famous temple walls of Angkor Wat. And in Egypt, archaeologist Sarah Parcak has used satellite imagery to uncover thousands of sand-buried structures and a hidden street grid at the ancient Egyptian city of Tanis.

Over the past seven years, I have followed archaeologists around ancient cities and pestered them with questions as I researched my latest book, Four Lost Cities: A secret history of the urban age. Many of these researchers call themselves data archaeologists because they need a lot of high-tech tricks to understand large settlements that had once been full of hundreds of thousands of people. When you are studying a city, it isn’t enough to dig up a few statues or baubles. You want to suss out the entire street grid, the distribution of bars and temples, the water infrastructure and even the number of public toilets. Tools like magnetometry and lidar give us the panoramic view we need to comprehend large-scale habitats.

But archaeologists use other tools too, like 3D photography, so that they can capture all the details of a site to study later in an air-conditioned office. They enter each finding into databases, from ceramic pot designs to the number of pubs on a street, looking for patterns that might reveal shared beliefs or cultural connections.

Studies like these led to the discovery that cities have always had significant populations of immigrants, even 9000 years ago. They also allow archaeologists to figure out what ordinary people did for fun. At Cahokia, for instance, people played a game called Chunkey with special stone pucks that researchers have catalogued up and down the Mississippi.

Perhaps most importantly for us today, data archaeology has helped us see trends in why people abandon cities. Most settlements take at least a century to empty out, but generally people start to leave when local government is unable to deal with climate disasters like drought. Once environmental troubles and political instability have festered for decades, the population slowly moves elsewhere. We urbanites have always voted with our feet. The question is where we will take our civilisations next.