A surprise in the deep ocean has turned into one of the biggest whale-fall finds on record.

Researchers from China, Italy and New Zealand reported in Nature on Wednesday that they found a whale “necropolis” in the southeastern Indian Ocean containing recently dead whales and fossils dating back at least 5.3 million years. The site is the largest, deepest and oldest whale fall recorded so far.

The authors wrote in their paper: “These findings reshape the understanding of the limits and biogeography of whale-fall ecosystems.”

Whale falls happen when a whale carcass sinks to the deep ocean floor. The body creates a burst of resources in the area, and cold temperatures and high pressure can keep it intact far longer than if it died elsewhere. At each stage of decomposition, the whale supports different forms of life, and even the mineral skeleton at the end acts as a scaffold for coral and other suspension feeders.

Around 70 whale-fall sites have been documented today, but much less is known about the ecosystems created by the deepest ones. Until now, the deepest natural whale fall on record was found in 2016 at 4,204 metres in the southwest Atlantic Ocean, and it involved a single whale.

The new site was found during a 2023 expedition to the Diamantina Zone, a rift valley formed by the breakup of Australia and Antarctica about 50 million years ago. The team had been searching for fluid activity in the hadal zone, the deepest part of the ocean.

Lead author Xiaotong Peng, a professor at the Institute of Deep-Sea Science and Engineering, told Gizmodo: “The expedition to the Diamantina Zone had multiple primary objectives, as we had a multidisciplinary team on board, including geologists, geophysicists, biologists, and environmental researchers. One of our major targets was to search for fluid activities in the hadal zone. So, finding this massive whale graveyard came as a complete surprise.”

During one dive, the team found whale fossils about 7,000 metres deep. After that, 32 dives mapped an extensive ecosystem built around dead whales.

“It’s a strip mall of whale falls,” Stephen Godfrey, curator of paleontology at the Calvert Marine Museum in Maryland, who was not involved in the research, told Gizmodo.

The graveyard stretches 1,200 kilometres along the sea floor. Researchers found at least five active whale falls, all in the sulfophilic stage, the third stage of decomposition, which can last for decades. The deepest active whale fall was at 6,789 metres.

They also documented 476 fossilised whales. Of the 25 collected samples that could be dated, the oldest was about 5.3 million years old, suggesting whales have supported this deep-sea ecosystem for at least that long.

Most of the remains, recent and ancient, belong to deep-diving beaked whales. The researchers said their dense skull bones likely helped them remain intact on the ocean floor. They also said the bones were often coated with ferromanganese oxides, which formed a protective layer that isolated them from seawater. Sediment also reaches the deep sea very slowly, allowing the bones to remain largely unburied for millions of years.

“Ironically, in life beaked whales are seemingly rare and elusive,” Godfrey said. “Here in death, they are relatively abundant because the skulls of male beaked whales are very densely ossified and consequently survive biological and chemical degradation.”

Godfrey said fossil mass graveyards are known from other places and are sometimes called Lagerstätten. In his editorial for Nature, he called this discovery the first known “Wachsend-Lagerstätten”, a growing fossil deposit, because living whales are still being added to it.

The researchers documented 35 broad groups of life, or taxa, around the whale falls. Most of the species within those groups likely have not yet been classified. The team has also identified and named at least one extinct whale species, Pterocetus diamantinae.

“Given the length of this fracture zone, there are likely thousands upon thousands of fossil whale skulls preserved therein,” Godfrey said. “Given that possibility, other new kinds of extinct beaked whales might be found. And in theory, one could document the diversity, abundance, and evolutionary history of local beaked whales through time.”

The researchers said similar whale necropolises may exist elsewhere.

Co-author Xikun Song, an associate professor at the Institute of Deep-Sea Science and Engineering, told Gizmodo: “Similar whale necropolises probably exist in other core beaked-whale habitats, such as South Africa, the Iberian Peninsula and off the Crozet and Kerguelen islands, as indicated by the recovery of abundant fossils by trawling, indicating that comparable hidden archives may be widespread in the global deep oceans.”

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