The ancient underwater ruins that were recently discovered in Greece may not actually have been a sunken city but rather stone formations that were created by a natural phenomenon.

In a study featured in the journal Marine and Petroleum Geology, researchers from the University of Athens (UoA) and University of East Anglia (UEA) found evidence that suggests the underwater city near the Greek island of Zakynthos is actually a group of stones that were shaped by a naturally occurring event in the ocean.

While these marine formations do appear like paved floors and colonnades similar to those found in ancient Greek cities, UEA professor Julian Andrews noticed that the ruins didn't include any other signs of human activity, such as pieces of poetry.

This missing feature is what inspired Andrews and his colleagues to investigate the site more closely to find out if it is indeed the ruins of a sunken city.

Through the use of X-rays, microscopy and stable isotope analyses, the researchers examined the texture of the submerged objects as well as their mineral content. They discovered that these were formed by a natural geological event.

Andrews said the doughnut and disk formations, such as those that looked like the bases of columns, are the result of the mineralization of seeps made of hydrocarbon. This can often be seen in both ancient and present-day seafloor settings.

The way some of these doughnut-shaped objects are lined up suggests that there may be an underwater fault that partially ruptured the surface of the seafloor. This fault may have also released gases, such as methane, into the ocean.

Andrews added that the microorganisms may have used the carbon included in methane as fuel to drive the oxidation of the sediment. This in turn changed the chemistry of the stones, creating a natural kind of cement known as concretion.

The cement that was formed in the Zakynthos ruins was made of a mineral known as dolomite, which can be found in sediments rich in microbes.

The stone formations were ultimately exposed through erosion, leaving them to be found by divers in the present day.

"These features are proof of natural methane seeping out of rock from hydrocarbon reservoirs," Andrews said.

"The same thing happens in the North Sea, and it is also similar to the effects of fracking, when humans essentially speed up or enhance the phenomena."

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