Scientists make surprise discovery of life in the seafloor’s ‘underworld’

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Scientists have uncovered communities of animals such as tube worms and snails living in volcanic caves beneath the seafloor, revealing a previously unknown but thriving ecosystem.

Researchers made the astonishing discovery during a 30-day expedition aboard the Schmidt Ocean Institute’s research vessel “Falkor (too)” to explore an undersea volcano off Central America that’s part of the East Pacific Rise.

A volcanically active ridge, the extensive rise occurs where two tectonic plates meet on the floor of the Pacific Ocean. Located along the ridge are hydrothermal vents, or openings in the seafloor where seawater and hot magma from beneath Earth’s crust come together to create a type of underwater hot spring.

A variety of sea life clusters around the vents, which belch out elements that help bacteria, mussels, tube worms and other animals survive at extreme ocean depths. The vent ecosystem has been studied in-depth, but areas beneath the vents have largely remained out of reach.

Using the remotely operated vehicle SuBastian, researchers exposed parts of the subseafloor and uncovered a surprise: caves connected to the vents teeming with giant tube worms, some reaching up to 1.6 feet (0.5 meter) long, and other animals. The revelation suggests connectivity between the seafloor and subseafloor ecosystems, allowing life to thrive in unexpected places above and below the ocean floor.

While the team first observed the subseafloor ecosystem in the summer of 2023, the research describing the environment and its animals published Tuesday in the journal Nature Communications.

“We want to understand how animals travel and how they disperse, so we looked for the first time into the subsurface,” said study coauthor Dr. Sabine Gollner, marine biologist and senior scientist at the Royal Netherlands Institute for Sea Research, in a video the Schmidt Ocean Institute released. “Animals are able to live beneath hydrothermal vents, and that, to me, is mind-blowing.”

Scientists have long been intrigued by the animal life that clusters around hydrothermal vents and have studied these unique ecosystems for the past 50 years.

The shifting of Earth’s tectonic plates gives rise to new hydrothermal vents over time, and foundational seafloor animals such as tube worms have been known to colonize these new vents in the span of a few years.

Microbial life exists beneath the seafloor based on samples from fluid released by the hydrothermal vents, some research has also suggested. And tube worms were observed living several centimeters deep within the cracks of the seafloor near vents, but the study team wasn’t sure how tiny tube worm larvae, less than 0.04 inch (1 millimeter) in length, would be able to swim against the current to settle and grow in these spaces, Gollner said. Tube worms are immobile creatures that settle and grow in one place without moving, like barnacles.

“That is why we hypothesized that tubeworm larvae can travel in cracks below the ground with the warm vent fluid to colonize the surface vents from below,” Gollner said in an email.

The research expedition, helmed by lead study author Dr. Monika Bright, professor and head of the department of limnology and bio-oceanography at the University of Vienna, designed an experiment involving the placement of mesh boxes on the seafloor 8,251 feet (2,515 meters) below the ocean’s surface to collect samples from cracks in Earth’s crust.

But it was more difficult than expected, Gollner said. Direct evidence of animal life beneath the seafloor would require more heavy lifting — literally.

“We kind of needed to be creative and apply basically a new method,” Gollner said. “Flipping around the rocks opened our view into the underworld of hydrothermal vents.”

The team used the SuBastian robotic explorer to drill small holes into rocks on the seafloor and lift them up. The ROV’s arm flipped over a small chunk of the volcanic crust, revealing cavities below the hydrothermal vents filled with water at a balmy 75 degrees Fahrenheit (24 degrees Celsius) — as well as tube worms in both larvae and adult form, mobile animals such as snails, and chemosynthetic bacteria.

Sunlight can’t penetrate the depths of the ocean floor to help organisms convert elements into oxygen and sugar through photosynthesis. Instead of sunlight, chemosynthetic bacteria use chemical reactions to produce sugars that other animals clustered around the vents use to live.

“Our understanding of animal life at deep-sea hydrothermal vents has greatly expanded with this discovery,” Bright said in a statement. “Two dynamic vent habitats exist. Vent animals above and below the surface thrive together in unison, depending on vent fluid from below and oxygen in the seawater from above.”

Uncovering life in this previously unknown subhabitat suggests there may be many more organisms than scientists have documented within the ocean’s depths or along its floor, said Alex Rogers, marine biologist and science director for Ocean Census. Rogers was not involved in the new study.

It’s also likely that these subterranean ecosystems remain once the vents become inactive over time, so they could form new habitats for other species, he said.

“The paper adds to our understanding of vent ecosystems, how populations of vent organisms are maintained and just how much life exists at these systems,” Rogers said.

Next, the team wants to determine whether life exists beneath all deep-sea hydrothermal vents as well as how far the caves extend horizontally and vertically, Gollner said.

“The discovery of animal life beneath the surface of the Earth’s crust raises questions concerning the extent of these ecosystems, which is larger than what can be seen on the seafloor surface,” the authors wrote in the scientific report. “The study of the subseafloor biosphere for animal life has just begun.”

But extreme care must be taken when studying these fragile ecosystems.

For the study, the researchers only lifted six small squares of the seafloor measuring about 20 by 20 inches (50 by 50 centimeters) to make as little disturbance as possible.

The team worries that lifting larger pieces or any form of major drilling, such as deep-sea mining, could change the route of hydrothermal vents and redirect them to be expelled at other locations, causing the animal life that clusters around the vents to die, Gollner said.

“With this understanding, we also know that we not only need to protect what we see on the surface, but also we should protect what is living below, because it is one important component of this ecosystem,” Bright said.