Patience Seamount is an underwater mountain in Australia’s Huon Marine Park off the coast of Tasmania. Every year, it is the site of an enormous gathering of basketwork eels (Diastobranchus capensis) for reasons unknown, although most likely to spawn. The numbers are huge, but they have never been quantified.

Now a team led by marine scientist Ben Scoulding of Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) has estimated how many are down there.

According to their study published in the journal Royal Society Open Science, the gatherings could contain as many as 2.8 million eels with a collective biomass of around 3,550 metric tons (about 3,900 U.S. tons).