Today all that’s left of the ancient city of Semiyarka are a few low earthen mounds and some scattered artifacts, nearly hidden beneath the waving grasses of the Kazakh Steppe, a vast swath of grassland that stretches across northern Kazakhstan and into Russia. But recent surveys and excavations reveal that 3,500 years ago, this empty plain was a bustling city with a thriving metalworking industry, where nomadic herders and traders might have mingled with settled metalworkers and merchants.

University College of London archaeologist Miljana Radivojevic and her colleagues recently mapped the site with drones and geophysical surveys (like ground-penetrating radar, for example), tracing the layout of a 140-hectare city on the steppe in what’s now Kazakhstan.

The Bronze Age city once boasted rows of houses built on earthworks, a large central building, and a neighborhood of workshops where artisans smelted and cast bronze. From its windswept promontory, it held a commanding view of a narrow point in the Irtysh River valley, a strategic location that may have offered the city “control over movement along the river and valley bottom,” according to Radivojevic and her colleagues. That view inspired archaeologists’ name for the city: Semiyarka, or City of Seven Ravines.

Archaeologists have known about the site since the early 2000s, when the US Department of Defense declassified a set of photographs taken by its Corona spy satellite in 1972, when Kazakhstan was a part of the Soviet Union and the US was eager to see what was happening behind the Iron Curtain. Those photos captured the outlines of Semiyarka’s kilometer-long earthworks, but the recent surveys reveal that the Bronze Age city was much larger and much more interesting than anyone realized.