Geologists know that there are two kinds of “lost cities” underwater: those that were made by humans, and those that weren’t. The latter turns out to be the case for one such city, discovered by tourists diving off the Greek island of Zakynthos in 2014. The divers snapped photos of what they thought looked like the remnants of a paved stone walkway and colonnades (above). After they uploaded the images to Google Earth, Greece’s Ephorate of Underwater Antiquities launched an underwater investigation. But no human artifacts—such as coins or pottery—were found at the site, making it increasingly unlikely that it was humanmade. Instead, the smooth structures were manufactured by microbes, scientists report online today in Marine and Petroleum Geology. Scientists examined the underwater terrain and levels of carbon, oxygen, and strontium isotopes in the structures to figure out how they formed. The answer? Sulfate-reducing bacteria in the sediments, which thrived on methane seeping up through the sea floor. The microbes’ activity altered the soil chemistry, creating favorable conditions for a type of magnesium-rich carbonate rock called dolomite to form. When the methane seeps are diffuse, the bacteria engineer sheetlike dolomite structures that resemble paving stones. When the seeps are more focused, the bacteria help create donutlike structures that look an awful lot like wheels. The process happened some 3 million years ago, and took perhaps hundreds to thousands of years—a blink of an eye geologically, but far longer than the time to build a human city.