See the first detailed illustration of an ancient deep-sea mountain

Rising mountain peaks and sharp rock formations cover the ocean floor, with corals, sponges, fish, and tons of sea creatures clinging to their sides. Unlike mountains on land that dare climbers and hikers to brave their peaks, underwater mountains, called seamounts, are vastly underexplored. Submerged at often unfathomable depths, many are inaccessible to even the most experienced divers. 

But now, a Boston University–led team of scientists has made it possible to view a deep-sea mountain—to study its geographic features and biodiversity—without even getting wet. It’s the first time a single seamount in the Pacific Ocean has been artistically visualized with this level of precision.

The team mapped a seamount located on the edge of the United States Exclusive Economic Zone in the central Pacific, producing a stunning illustration depicting corals and deep-sea organisms that live among complex ocean currents. The mountain, which sits a couple thousand miles southeast of Hawaii and is also on the border of the Phoenix Islands Protected Area, was brought to life using data collected during an expedition led by Randi Rotjan, a Boston University College of Arts & Sciences research associate professor of biology.

To do so, Rotjan and her team used a remotely operated vehicle (ROV), which scaled the seamount four times to illuminate the abundance of corals and deep-sea organisms that call it home at different depths. They then leveraged that data to inform an artistic rendering in the style of Alexander von Humboldt, a revered 19th-century naturalist who was one of the first to depict complex natural systems artistically. Rotjan says that Humboldt’s legacy inspired her and the scientists to commission Constance Sartor, artist-at-sea and marine scientist at Schmidt Ocean Institute.

“The deep sea is a dark, impenetrable place where you can’t take satellite views or landscape images,” says Rotjan. “We need art to help us visualize the deep sea, and we drew inspiration for this piece directly from Alexander von Humboldt.”

The seamount is part of an ancient system of underwater volcanoes dating back over 60 million years. Most of the mountain is shrouded in darkness, but the top sits 196 meters (643 feet) below the surface within the mesophotic zone—also known as the ocean “twilight zone,” where a small amount of sunlight allows corals and algae to photosynthesize. Below the twilight zone, deep-ocean corals don’t need light to thrive. The team collected ROV data from the top down to 1,500 meters (4,921 feet) deep, documenting over 100 different types of coral living on all sides of the seamount. The base is about 5,000 meters (16,400 feet) below the surface, making it about two and half times the size of Mount Washington, the tallest peak in New England. The team published their findings, and the artistic rendering, in Scientific Reports. 

“There’s very little work that’s been done to explore the deep sea. Less than 1 percent of the ocean seafloor has actually been imaged globally,” says Brian Kennedy (CAS’23), who earned his PhD in the Rotjan Lab and is lead author of the study. The paper was first published as a chapter in his PhD dissertation. He is now the chief scientist at the Ocean Discovery League, a nonprofit focused on expanding deep-sea science. 

The Brink spoke with Kennedy and Rotjan to learn all about the newly documented seamount, the importance of deep-ocean corals, and how the deep sea is connected to our lives on land.

Q&A with Randi Rotjan and Brian Kennedy:

The Brink: What exactly is a seamount, and why did you decide to focus on this one?

Rotjan: A seamount is an underwater mountain that has all of the things that mountains on land have, but instead of wind and precipitation, it has ocean currents that carry oxygen and nutrients. For this study, we wanted to get a sense of the ecological rules that govern underwater mountain biology. On land, the patterns on a mountain are obvious—the top has low tree cover, then there’s a conifer zone, and then the deciduous zone. But in the deep sea, we don’t know any of this because we haven’t seen most of the deep sea.

Kennedy: When we think about deep-sea science, a lot of the questions we’re asking about the deep ocean are the same questions many great explorers and natural historians were asking about nature centuries ago. Thinking about seamounts, and how analogous they are to terrestrial mountains, we don’t understand the processes that control the distribution of life the same way we do on land. This work was inspired by trying to get a better understanding of the bigger picture of what shapes biodiversity and community composition on seamounts. We wanted to take a deeper dive on a seamount that was unexplored.

What was it like to document a previously unexplored area in such a remote part of the world?

Kennedy: In the beginning, there was some uncertainty around this project since it [started] right after COVID, and we really weren’t sure it was going to happen. In some ways, it was a miracle we pulled this expedition off at all. We went to sea with less of a rigid cruise plan than normal. Once we mapped this area using multibeam sonar, we realized it was a great candidate. The summit of the seamount reaches the mesophotic zone, meaning that it receives enough light for photosynthesis to occur. It’s also relatively symmetrical, and exists in some really complex equatorial current structures. There are really strong currents at different depths that change direction.

Rotjan: It was the perfect seamount for what we were looking for. We decided in real time as a team to perform four [ROV] dives at the seamount, which is no small thing. Each dive takes a lot of time and resources, and are each 24 to 36 hours long, so spending that much time and attention on one seamount is rare. We had our artist, Constance, onboard the vessel with us.

How do the ocean currents influence the seamount and the life that exists there?

Kennedy: In the deep sea, we don’t understand the full relationship, but we have some ideas. The assumption is that [ocean] currents are better for deep-sea coral diversity. On the equator, there are complex current systems that change at different depths and vary dramatically in direction and speed. The summit of this seamount reaches the equatorial undercurrent, which is one of the most powerful currents in this region of the Pacific. We saw four distinct current systems on the seamount: a very strong west to east, a weaker west to east, a reverse east to west, and at the deepest depths, we saw a south to north flow. We hypothesized that each one would host different communities—but, of course, the answer was more nuanced than we were expecting. Corals were pretty consistent on all sides—some were affiliated to currents, but others didn’t match the current patterns.

Rotjan: Once we start to understand some of these patterns and answer these questions, scientists can know if an abundant coral on the north side, like Swiftia, is found in any unusual spots that would be noteworthy. We can start to unpack the biodiversity. Since we haven’t been able to do this before at this scale, this is so exciting.

I know another important factor is oxygen, and you were able to document the oxygen minimum zone, or OMZ—pockets where there is less oxygen available than the surrounding water—on the seamount. Can you explain the significance of that?

Kennedy: Oxygen minimum zones exist in every ocean, which is where oxygen is lower than the rest of the water column. One of the cool things about this particular seamount is that there are two minimum zones—one at 200 to 300 meters, and another one around 500 to 600 meters. Studying oxygen minimum zones is important because climate change is going to have a really major effect on oxygen distribution. A lot of the climate modeling indicates that oxygen concentrations in the deep ocean are going to decrease, and oxygen zones are going to expand dramatically in some parts of the world. We have to understand how sensitive different corals are to low oxygen states and how long they can sustain in a lower oxygen state before they start dying.

Rotjan: Even though there are oxygen minimum zones all over the ocean, this particular finding shows the stability of the zone on this seamount, because if it changed frequently, these corals would have a much bigger gap between them. These species are able to live on either side of the divide, but not in it, which begs all kinds of questions, like, is it the same population above and below the OMZ? Can coral larvae move through the oxygen minimum zone? We know climate change can also slow down or disrupt ocean currents, which can change the oxygen minimum zones and current dynamics, so disrupting the currents disrupts all of those systems.

Why is it important to learn more about deep-sea corals?

Kennedy: Deep-sea corals and sponges are ecosystem engineers in the environment. Deep-sea communities play an important role in increasing nutrients concentration in deep water, which is one of the reasons upwelling areas are so productive. That water becomes nutrient-rich during its time in the deep sea by bacterial processes that corals and sponges have a heavy effect on modifying. The ocean around the world is changing very fast, and we need to know how to understand the impacts of climate change and human activities.

Rotjan: There is incredibly high biodiversity in seamounts, and these equatorial seamounts have some of the highest seamount biodiversity there is. One of the other key pieces about these corals is that they grow very slowly. These corals can be 4,000 years old—they’re old on a different level. They’ve lived through thousands of years of human history. Are we going to let them die on our watch? No.

How do you hope that this discovery can steer the conversations about deep-ocean conservation?

Rotjan: This seamount is right on the edge of the US Exclusive Economic Zone, in an unprotected area just outside of the recently renamed Pacific Remote Islands—now the Pacific Islands Heritage—Marine National Monument. There have been calls to expand protections in these US waters to include this seamount, and the many others in the area. Protecting oceans should be a bipartisan issue, and there has been historic bipartisan support for protecting these special, yet vulnerable, places.

Kennedy: We need to think about individual seamounts as much more complex entities than we have done previously. Seamounts that host photosynthetic communities—but don’t break the surface—are very rare globally. I hope this paper opens the door to more people recognizing how special these oceanic features are.

This interview was edited for length and clarity. 

This work was supported by the National Oceanic and Atmospheric Administration and the Schmidt Ocean Institute.

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