The Mystery of the Sunken City of Zakynthos

This scientific discovery reads like a science-fiction mystery novel.

It starts in 2014, when a group of tourists visiting the Greek island of Zakynthos decided to go snorkeling. While exploring the shallow ocean waters close to Alikanas Bay they stumbled upon what appeared to be the remnants of a sunken civilization: flat pavement stones and the circular bases of collapsed colonnades. Perhaps these were lost fragments of a city port submerged by the sea?

The divers took pictures and uploaded them online, where they reached the attention of Greece’s Ephorate of Underwater Antiquities, who sent in an archaeological investigation team. The team discovered something very odd: there was no pottery, no coins, no signs at all of any human life. If this had once been a city, who lived there? Who built it? And why didn’t they leave behind any artifacts?

Is this the base of an ancient colonnade? If so, who built it?
And what happened to the rest of the city? The answer is surprising.
Image credit: University of Athens.

So the scientists decided to dig deeper. They analyzed the minerals in the structures to find clues to their origins. Amid the chemical analyses, X-rays, and microscope images, they found their answer: these structures were not hundreds of years old, but millions. And they weren’t built by humans, but by bacteria.

Deep below the ocean’s surface, on the seafloor below the reach of the sun’s rays, life would seem impossible in the absence of sunlight. Yet in some places, natural gases such as methane seep up from beneath the sediments of the abyss, and from those gases, microbes find the fuel they need to survive. These cold seeps can famously provide the foundation for entire deep-sea ecosystems that thrive without any photosynthesis at all.

(Check out this video of cold seep ecosystems by Marum TV!)

As microbial bacteria and archaeans metabolize the emerging gases, the chemical reactions they perform change the chemistry of the ocean sediments in which they live. This can cause the sediments to form natural cements called concretions. The microbial colonies may form flat mats, which would leave behind sheets of cemented soil, or in places where gases are concentrated colonies may surround the gas seeps and form circular or doughtnut-shaped concretions.

And that is exactly how the “Lost City of Zakynthos” was formed.

The “pavement” and “colonnades” are made of a mineral called dolomite, and the chemical isotopes within them show the tell-tale signatures of methane-metabolism. This comes as a bit of a surprise to researchers: these structures are only a few meters underwater, whereas most cold seeps are found near subduction zones deep beneath the sea. In this case, scientists suspect an underground fault line allowed gases to escape in this region, allowing for the formation of an unusually seep ecosystem.

Unlike human builders, it would have taken these microbial architects centuries or millennia to construct these Hellenic look-alikes. Their stonework features no windows or doors, and of course no pottery, merely a set of shapes familiar enough to make for a perplexing mystery with a fascinating natural explanation.

This image shows a modern-day bacterial mat at a seep in the
Western Atlantic Ocean. Image from Wikimedia Commons.

References: 

Andrews et al. 2016. Exhumed hydrocarbon-seep authigenic carbonates from Zakynthos Island (Greece): Concretions not archaeological remains. Marine and Petroleum Geology.