A group of snorkelers swimming near the Greek island of Zakynthos noticed something bizarre as they floated in roughly 20 feet of water. According to a report from the Washington Post, what they initially believed to be the ruins of an ancient city turned out to be something completely different.

A study published in the journal Marine and Petroleum Geology reveals that the bizarre structures with a surprising level of symmetry was not created by humans, but was in fact the work of bacteria.

A team of researchers from East Anglia University began investigating the area after the snorkelers reported the find. While the size and shape of the structures certainly appear to be the ruins of forgotten buildings and roads, archaeologists immediately noticed the lack of other signs of human activity around the site.

According to co-author Julian Andrews from the University of East Anglia, “Essentially what you’ve got are bacteria that are fossilizing the plumbing system.” Andrews explained that the structures were made of dolomite, the byproduct of bacteria feeding off of the methane that leaks from the sea floor.

Over the years, as the bacteria gathered around a leaking jet of methane at the site, the calcium byproduct accumulated to create some pretty fascinating structures. The size and pressure of the methane jets would have influenced the appearance of the structures; those made by small, focused methane jets look like tall tubes jutting off of the sea floor, while large flat slabs were likely made by wider, burping methane leaks.

Scientists estimate that the methane jets stopped leaking some time around the Pliocene era, which began 5.3 million years ago and ended 2.6 million years ago. There isn’t any methane leaking from the sea floor in the area today, but the bizarre structures are a reminder of the constantly changing nature of the Earth.

A press release from the University of East Anglia describing the details of the study can be found here.