Around 1,000m deep, researchers found a paradoxical site where eels thrive but everything else succumbs to its death trap. When they returned, it had vanished
The idea of being surrounded by hundreds of slithering eels is enough to make most people squirm. But that’s exactly what researchers found in the deep waters of the Pacific Ocean.
In 2005, scientists visited a 4,360-metre underwater volcano called Vailulu’u around 1,000m deep in the Samoan Island archipelago. Its base stretches out for more than 20 miles and its crater at the summit is around one mile wide.
This seamount had been mapped by researchers on a Woods Hole Oceanographic Institution (WHOI) expedition in 1999 and the new expedition aimed to find out more about this unusual place.
The experts got more than they bargained for. Not only did they find Vailulu’u but they noticed a 300m volcanic cone in its crater. It hadn’t been there when scientists had visited the volcano just four years before. They named it Nafanua.
“Nafanua grew from the 1,000-m-deep crater floor in <4 years and could reach the sea surface within decades,” the scientists wrote in their study. Its rim rises up to shallower waters of around 590m deep.
It seemed like an inhospitable site: iron oxide poured into the water from hydrothermal vents, creating layers of up to one metre thick. Elsewhere, high-temperature vents of around 81°C spewed out acid and what may have been liquid carbon dioxide.
“Vents fill Vailulu’u crater with a thick suspension of particulates and apparently toxic fluids that mix with seawater entering from the crater breaches,” says the study.
Despite its strange features, the scientists were surprised to find life thriving in these deep waters. eels (Dysommina rugosa) were swimming around Nafanua’s summit in their hundreds.
The researchers hypothesised that currents were bringing midwater shrimp to the summit and the eels had arrived to feast on them. Other animals weren’t as lucky. “The moat and crater floor around the new volcano are littered with dead metazoans [multi-cellular animals] that apparently died from exposure to hydrothermal emissions,” they wrote in the study.
Marine worms (polychaetes) that could tolerate the acidic condition were able to survive, feeding on carcasses, but everything else seemed doomed.
“Although eels thrive in hydrothermal vents at the summit of Nafanua, venting elsewhere in the crater causes mass mortality,” according to the study. “Paradoxically, the same anticyclonic currents that deliver food to the eels may also concentrate a wide variety of nektonic [free-swimming] animals in a death trap of toxic hydrothermal fluids,” it says.
Bizarrely, when scientists returned to the site in 2017, they couldn’t find the vents. They spotted a handful of solitary eels from their remotely operated vehicle (ROV) but not the bustling city that had been described in the previous study.
“Inaccurate navigation data could explain this finding, or it may suggest that the site was either paved over by lava by 2017 or became extinct when fluid flow ceased,” say the scientists in a research paper documenting their exploration of the Vailulu’u submarine volcano.
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The hydrothermal vents spotted by the submersible in 2005 could have been because the volcano had only recently erupted. By the time of the 2017 mission, the rock may have cooled, leaving no sign of the mysterious Eel City.
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Top image: NOAA Photo Library, Public domain, via Wikimedia Commons
