What looked like a long lost Greek city on the bottom of the sea back in 2013, has now been deemed as "a naturally occurring phenomenon."
The underwater divers, who discovered what looked like paved floors, courtyards and colonnades, thought they had found the ruins of a long-forgotten civilization that perished when tidal waves hit the shores of the Greek holiday island Zakynthos.
But the research from the University of East Anglia (UK) and the University of Athens (Greece) revealed that the site was created by a natural geological phenomenon that took place in the Pliocene era - up to five million years ago.
The bizarre discovery, found close to Alikanas Bay, was carefully examined in situ by the Ephorate of Underwater Antiquities of Greece.
Lead author Julian Andrews said, "We investigated the site, which is between two and five meters under water, and found that it is actually a natural geologically occurring phenomenon. 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."
Andrews noted 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.
In this case the cement was an unusual mineral called dolomite which rarely forms in seawater, but can be quite common in microbe-rich sediments. These concretions were then exhumed by erosion to be exposed on the seabed today, she added.
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She concluded that these features are proof of natural methane seeping out of rock from hydrocarbon reservoirs. 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.
The study appears in Marine and Petroleum Geology.