It is also the deepest and oldest known whale graveyard on Earth, according to research published in the journal Nature on Wednesday, with some fossils dating back 5.3 million years.

From inside a small submersible, the Chinese researchers saw an array of strange animals -- many believed to be new to science -- living off the whale carcasses.

A new, though extinct, species of whale was also identified among the nearly 500 skeletons found up to 7,000 metres deep along a 1,200 kilometre corridor of bones in the Indian Ocean west of Australia.

Lead study author Xiaotong Peng of the Chinese Academy of Sciences told AFP that the researchers were "astonished" when the scale of their discovery became clear.

It was known that when whales die and drop to the seafloor, their sunken bodies -- called "whale falls" -- provide a source of food to bottom-dwelling creatures.

"But discovering a necropolis of this scale was completely unexpected: the size of distribution, the depth and the age range were far beyond anything we had imagined," Xiaotong Peng said.

The researchers had several theories for why so many whales died in this particular corridor, including that it is a popular foraging area and has a V-shaped trench that funnels carcasses to the ocean's floor.

- 'Truly incredible experience' -

For the discovery, the Fendouzhe submersible carried out 32 dives in 2023 -- though what it found was only revealed in Nature on Wednesday.

The sub took up to three people on the dives, collecting the fossil samples using robotic arms.

Study co-author Peng Zhou said witnessing the whale graveyard "was a truly incredible experience".

"The vibrant ecosystems we saw offered a completely different perspective on this otherwise dark and cold ocean floor."

Among the animals they discovered living off the carcasses were jellyfish, worms, snails, crustaceans, brittle stars and molluscs called bivalves.

Extrapolating from the number of bones they found, most of which were from beaked whales, the scientists estimated there could be more than 10 million carcasses across the area called the Diamantina Zone.

The soft tissue and lipids inside that many carcasses "translates to roughly 6.7 million tonnes of sequestered carbon," Xiaotong Peng said.

This provides an immense source of sustenance for animals, similar to how hydrothermal vents create their own ecosystems on the ocean floor.

Some of the animals seen by the scientists also live in hydrothermal vents and cold seeps, suggesting whale carcasses could help connect these deep-sea communities to each other.

While this is by far the largest whale graveyard yet found, fossils found during trawling suggest there could be others off South Africa, the Iberian peninsula and the Crozet islands, according to the study.

- 'More blockbusters to come'? -

University of Hawaii oceanographer Craig Smith, who discovered the first whale fall in 1987 but was not involved in the new research, told AFP it was "extremely exciting".

"The vast number of fossil whale falls documented, including a new species of beaked whale, is truly amazing and is of major importance to understanding whale evolution and whale distributions over geologic time," he said.

Whale fall researcher Amy Baco-Taylor at Florida State University told AFP the "remarkable discovery" would "likely provide many new insights".

"It does seem very strange" that so many whales died in this area, Baco-Taylor admitted, adding that "we don't know enough about whale consciousness".

US palaeontologist Stephen Godfrey compared the "truly unique discovery" to past major underwater finds, such as when scientists first identified hydrothermal vents teeming with life on the ocean floor in 1977.

He called for future submersible voyages to find more whale graveyards across the world.

This discovery "reminded me of a trailer for the first in a series of epic movies", Godfrey commented in a linked Nature paper.

"I hope that there will be many more of these blockbusters to come."