2.8 Million Eels Gather at One Underwater Mountain and No One Knows Why

Every southern autumn, in cold dark water far below the surface off southeast Australia, millions of eels crowd around a single small underwater mountain. Scientists can film them, count them and weigh them, but they cannot explain why this one mountain, out of all the similar ones nearby, gets the crowd.

A swarm that fills the water

The mountain is called Patience Seamount, and it sits south of Tasmania. A study led by marine scientist Ben Scoulding of Australia's CSIRO worked out the scale of the gathering for the first time. At its peak, the researchers estimate, as many as 2.8 million basketwork eels could be present, together weighing more than 3,500 metric tons, or about 3,860 US tons.

The swarm is not a thin cloud of fish. At its largest it stretched roughly 3 kilometers long, 2 kilometers wide and 150 meters above the seafloor. That is about 1.9 miles by 1.2 miles, and 492 feet high. The mountain itself is tiny by comparison: Patience Seamount covers only about 2 square kilometers, or 0.8 square miles.

The crowd also grows through the season. The weight of eels rose from around 750 metric tons in March to more than 3,500 metric tons in May.

One mountain out of about 120

Patience is one of around 120 seamounts in the Huon and Tasman Fracture marine parks south of Tasmania. A seamount is simply an underwater mountain. Nearby seamounts sit at similar depths and share broadly similar conditions, yet they do not host the same gathering of eels.

"That's one of the biggest unanswered questions," Scoulding said. "At this stage, we have some good hypotheses, but we don't yet have a definitive answer."

The eels have been noticed for a long time. Commercial fisheries reported unusual numbers in the area since at least the 1980s and 1990s, but it was a deep-sea camera survey in 2007 that finally filmed the gathering. That survey was part of an expedition in the Huon Marine Park, which was established in 2007 as an expansion of an earlier marine reserve created in 1999 to protect the region's deep-sea life.

Probably a spawning crowd

The eels live around the seamount all year, but their numbers peak in the southern autumn, from March to May. Eels caught during that peak have been found in advanced spawning condition, both male and female. Spawning means releasing eggs and sperm, so this strongly suggests the whole gathering is about reproduction.

Counting the animals is not simple. Sonar can map dense groups of fish in the water, but it cannot tell one species from another, and orange roughy also gather at Patience Seamount. So the team combined sonar readings with underwater camera footage to work out how much of the signal came from each species.

An earlier study, published in 2021, used acoustic measurements and counts from camera images to estimate that tens of thousands of eels may have been present during a gathering in April 2015. The newer surveys, carried out between 2023 and 2025, pushed the estimate far higher.

What the eels are doing there is now fairly clear. Why they choose Patience Seamount, and not one of the many other mountains like it, is still open.