Tourists snorkeling near the Greek isle of Zakynthos have found mysterious structures in 20 feet of water that some think could be a lost city — and scientists now have an explanation for them.

The structures look like cobblestones and cylinders built in the style of the ancient Greeks, but a new study finds that it’s not a city at all — incredibly, the structures were entirely built by bacteria, according to a University of East Anglia statement.

After hearing the report from tourists, archaeologists dove down themselves to examine the area and noticed that while the structures were interesting, there was a complete lack of any evidence that humans were ever there — for example, no coins, pottery fragments or anything else that you would usually find in an ancient settlement.

So they started examining other possibilities, looking at the mineral content of these so-called structures and found that there is lot more there than meets the eye.

The structures have a mineral called dolomite, which is a byproduct of microbes that consume methane. Basically, the bacteria cluster around sources of gas, and their excrement produces a substance similar to cement.

So what the tourists were looking at wasn’t a lost city, but rather signs of gas seeping through subsurface faults, resulting in cluster of microbes leaving behind calcium deposits.

“We investigated the site, which is between two and five meters under water, and found that it is actually a natural geologically occurring phenomenon,” Lead author Prof. Julian Andrews, from UEA’s School of Environmental Sciences, said in the statement. “The disk and doughnut morphology, which looked a bit like circular column bases, is typical of mineralization at hydrocarbon seeps – seen both in modern seafloor and palaeo settings. We found that the linear distribution of these doughnut shaped concretions is likely the result of a sub-surface fault which has not fully ruptured the surface of the sea bed. The fault allowed gases, particularly methane, to escape from depth. Microbes in the sediment use the carbon in methane as fuel. Microbe-driven oxidation of the methane then changes the chemistry of the sediment forming a kind of natural cement, known to geologists as concretion.”