When a diver found a collection of rocks in a uniform pattern off the coast of the Greek island of Zante he thought he’d stumbled on the ruins of an ancient city. Now scientists have analysed the rocks and found that they were actually formed naturally.
The Ephorate of Underwater Antiquities of Greece examined the site, and decided to call in the experts to investigate. So Julian Andrews, a professor of environmental sciences at University of East Anglia and lead author on a paper published today in the journal Marine and Petroleum Geology, and colleague Michael Stamatakis, a professor of industrial minerals at the University of Athens, had a look.
"We quite quickly realised that these were, essentially, natural cementation of seafloor sediments," Andrews told BuzzFeed News.
The process starts when methane gas seeps up through sediments and vents. “Just before it hits the seabed, the bacteria get hold of this methane, which is fuel from their point of view,” said Andrews. “They start a chemical chain reaction within the sediment and that causes the formation of a mineral cement which is what we see in these structures."
Andrews said the rocks are no more than 3 or 4 million years old, although they could be younger. The scientists used various techniques to work out what minerals the rocks were made from and what bacteria had been involved in their formation.
“There's really no other explanation for what we see," said Andrews.
Sanjeev Gupta, a professor of earth science at Imperial College London who was not involved in the study, told BuzzFeed News: "I can see from the images that people could interpret those features as human-made, but the authors’ arguments are very strong, and they demonstrate very nicely that these are geological features.”
This explanation also accounts for why the structures are circular, and why there are ridges on some of them. "It's coming up like a pipe, and then the gas is diffusing out sideways as well as upwards, and it's that sideways diffusing which is quite regular that gives the circular, column-like structure," said Andrews. "How much gas moves sideways depends on how porous the sediment is, so you get those ridges that you see on the edges of the circular structure as well."
These kinds of structures are seen in many places over the world, often in places where gas is seeping up through the seabed. “The Earth’s crust is a very leaky reservoir – it's not very good at holding gas in,” said Andrews.
Bramley Murton, a researcher at the National Oceanography Centre in Southampton who was not involved in the study, told BuzzFeed News: "These sorts of phenomena occur in what we call plate convergence zones, where you have tectonic plates pushing together. There, you get thick piles of sediment forming through that process, and then you quite often get these structures forming too.”
But he said it’s “a little bit unusual” to find them in shallow water – where snorkellers can come across them – so they’re not often mistaken for the ruins of lost cities.
"It's very tempting to see regular-shaped rocks and interpret them as being manmade because they're so regular," said Murton. "We think that nature wouldn't produce such straight lines and right angles and so we like to jump to conclusions about their origins, and we forget that nature can produce a lot of very regular-shaped things.
"But the natural world can produce the most complicated and astonishingly geometric patterns just from natural processes.”