What Is Eel City, the Volcano Summit Crowded with Eels in the Deep Pacific?

Eel City is the nickname researchers gave to the summit of Nafanua — a cone nearly 1,000 feet tall that grew inside the crater of Vailulu’u, an active undersea volcano off American Samoa — after discovering hundreds of pale, ribbon-like eels packed densely around its peak, roughly 1,900 feet below the ocean surface.

Where Is Vailulu’u, and Why Does It Matter?

Vailulu’u sits hidden beneath the South Pacific, about 28 miles east of Ta’ū island in the Samoan chain. It is the youngest volcano along the Samoan hotspot — the same deep mantle plume responsible for building the Samoan islands over millions of years as the Pacific Plate slowly drifted over it. From the surrounding seafloor, Vailulu’u rises approximately 14,000 feet, making it as tall as many peaks in the Rocky Mountains, yet its summit never breaks the surface. Most of the world has no idea it exists.

How Fast Did the Nafanua Cone Grow?

Inside Vailulu’u’s summit crater, researchers discovered that a new inner cone had formed and grown to nearly 1,000 feet in fewer than four years — a rate of volcanic construction that startled the scientific community. The cone was named Nafanua, after a Samoan goddess of war. Follow-up mapping expeditions found that by 2019, the crater looked largely unchanged from 2017, suggesting the most dramatic phase of growth had already passed. Whether Nafanua will one day breach the surface and become a new Samoan island remains an open question scientists are actively watching.

Why Are There So Many Eels at the Summit?

The dense congregation of eels at Nafanua’s peak is genuinely strange, and researchers do not have a definitive explanation. The leading hypothesis is hydrodynamic: currents deflected upward by the cone’s shape concentrate shrimp and planktonic crustaceans at the summit, essentially delivering a steady food supply. The eels take shelter in crevices in the lava and feed on what the current brings to them. Hundreds of individuals pack the site so tightly that “Eel City” became the only reasonable name. It is one of the most unusual animal aggregations ever recorded on the deep seafloor.

What Makes the Crater Moat So Deadly?

While the cone’s summit supports a thriving community, the moat surrounding it on the crater floor — roughly 3,100 feet down — is a very different environment. Hydrothermal vents there discharge water at up to 178°F with a pH as low as 2.7, close to the acidity of vinegar. The vents also release iron-rich fluid and what researchers believe are rising droplets of liquid carbon dioxide. Fish that drift into this acidic brew do not survive. Their carcasses accumulate on the crater floor, where acid-tolerant worms move in to scavenge them. Life and death are separated here by just a few hundred feet of water column.

What Does Vailulu’u Tell Us About Deep-Sea Volcanism?

Vailulu’u is a reminder of how much geological and biological activity unfolds completely out of human sight. A volcano built a skyscraper-sized cone in under four years on the Pacific seafloor, and we only know about it because of targeted research expeditions equipped with remotely operated vehicles and multibeam sonar. The Samoan hotspot chain represents millions of years of plate motion recorded in volcanic rock, and Vailulu’u is its most current chapter — still being written, one lava flow at a time.