ON A FEBRUARY MORNING at Año Nuevo Reserve on the coast of Northern California, hundreds of gray-brown elephant seals lay strewn, lumplike, all over the beach. It was hard to hear anything over the honks and shrieks of status jockeying and sex dramas. The north wind gusted to 35 knots and blew a river of fine sand into everyone's faces. At a dense seal cluster near the water, researchers and students from the University of California, Santa Cruz, knelt over an 815-pound female whose stern had been labeled "X1" with Clairol bleach.

X1 was everyone's least favorite seal because she had a penchant for abandoning her pup to chase researchers. But she was zoned out for the time being, sedated for the hour they needed to sample her tissue, snip a whisker, swab her butt, and glue little boxes to her.

 Over the past month, X1 had birthed, fattened, and weaned her pup, all without eating or drinking a thing, as per the seal usual. Then she had gone into estrus and probably mated, though it could be months before she implanted the embryo to make a pup. Having completed these shore to-dos, she would soon pop off to the Pacific for a solo trip.

"Heads up!" UC Santa Cruz distinguished biology professor Daniel Costa called out from the sidelines. Costa has run the seal research lab for decades and directs the university's Institute of Marine Sciences. Today he was mainly acting as guide and bouncer. A young male, weighing about a ton, was galumphing toward them. One woman jumped up like a matador to brandish a bright-red scrap of tarp in the face of this aggrieved seal. He backed down. No one else in the X1 huddle looked up. They had a lot of work to do.

About three hours later, X1 woke up and waddled out to sea, joining an internet of animals—a network of seals across the planet that are sending us a constant stream of intelligence on the ocean.

THE FEMALE ELEPHANT SEAL is an ideal data collector. (Males have a persnickety physiological reaction to injections that makes them more difficult to sedate.) Unlike some other species of seals, elephant seals don't have to be caught, because they don't give a flying fish about humans and because they are loyal to their natal beaches, where they mostly lie around and bark at other seals. Every year, they undergo a "catastrophic" molt, shedding their outer skin and fur all at once, which gives the monitoring equipment that researchers were gluing to them this morning a humane natural expiration date.

Until then, whenever X1 took a breath (on average, once every 23 minutes), the tag atop her head would attempt to transmit data on the ocean conditions around her to a satellite. The data would be integrated, nearly in real time, into a network called AniBOS, short for animal-borne ocean sensors. From there, it would join the World Meteorological Organization's giant pool of ocean observations—which form the basis of modern climate models, the weather forecasts we all rely on, and long-term studies illuminating the ocean's future.

THE OCEAN IS VAST, and oceanographers are used to having a study subject that is way too big for them to get a good look at. So they make statistical models and use data collected in specific areas to see whether those models are right. The Argo network of buoys, a collaboration of some 30 countries, is one such data source. It consists of about 4,000 floats, each with an antenna resembling a four-foot-long hypodermic needle. Every 10 days, the buoys sink to more than 6,500 feet and then rise to the surface, collecting data on temperature, salinity, and depth along the way and transmitting it to a satellite. Argo is an extraordinary resource, but the buoys go only where ocean currents take them. And they mostly don't work in shallow or ice-covered water, which leaves big data gaps in tropical and polar regions.

Researchers can also collect ocean data by casting a rack of instruments—a contraption called a rosette—off the side of a research vessel, where it travels down into the abyss collecting measurements throughout the water column. The process costs around $40,000 a day, not including crew time, and you wouldn't want to do it in Antarctica in the winter.

In comparison, a female elephant seal swims up to 9,000 miles a year—some of X1's peers have gone nearly all the way to Japan—and regularly dives 3,000 feet into the twilight zone of the ocean, a depth we know almost nothing about, in search of fish and squid. A seal's tag collects an ocean profile, a series of snapshots of the ocean around her, every time she dives. It costs about $7,000, and it's reusable if she comes back with it.

"Seals are traveling all over the Amundsen Sea, right through the winter," said Mike Fedak, an emeritus professor of biology at the Sea Mammal Research Unit of the University of St. Andrews, in Scotland. "It's absolutely impossible data to collect in the standard way."

AniBOS is the effort of a group of some 40 researchers from pole to pole, led by biologist Clive McMahon at Australia's University of Tasmania and oceanographer Fabien Roquet at Sweden's University of Gothenburg. Since 2007, they have tagged an average of 100 animals per year—it's mostly seals for now, but sharks, whales, sea turtles, and seabirds are among the animals in the avant-garde. The three pounds of tags on X1 are 0.4 percent of her half-ton weight and won't slow her roll. AniBOS members must follow ethical guidelines that allow seals to be tagged only because the data they collect also benefits them and their fellow animals.

When an elephant seal is at sea, everything she does is of interest to biologists: what she eats, how fat she gets, where she goes, what she sees, what stresses her out. But the intel collected by AniBOS, even at this early stage, has already changed how we understand the ocean. Southern elephant seals have prompted epiphanies in our understanding of the world's currents. Northern ele­phant seals like X1 provided the bulk of the subsurface data on "the Blob," the mysterious and shocking plume of warm water that showed up in the North Pacific a few years back and ravaged much of the ocean life it encountered.

Roxanne Beltran, a professor at UC Santa Cruz, calls the seals "climate sentinels," though she often refers to them as "research partners" in conversation. If you want to know how hot it's going to get, how much the sea will rise, how strong the storms will be, what will happen to marine life, and which parts of the ocean are the highest priority for protection, elephant seals will help find the answers.

WHEN I TRACKED DOWN MIKE FEDAK, one of the first scientists to see the possibilities of satellites and seal tags, he was at a research station in Scotland's Shetland Islands. At 79, he had been drafted as a fieldworker for a harbor seal tagging project that was short on crew after a COVID-19 outbreak. Harbor seals are much sneakier than elephant seals, and Fedak would have had his hands full today, but it had been blowing too hard to take the boat out. So he had time before making coleslaw for the crew's dinner to take me through some of the challenges he and his peers had faced in enlisting animals as oceanographers.

Fedak has spent his career investigating the energetics underlying evolution. As a postdoc at Harvard, he ran ostriches and button quails on treadmills. As a new hire at the British Antarctic Survey's Sea Mammal Research Unit in the early 1980s, he looked at whether gray seals were really scarfing down fishermen's prey—and if not, what they were eating. For a seal, fat is life. The mothers melt away as the pups balloon up. How did females get fat enough to feed their pups?

"We knew absolutely diddly about what the seals did—well, almost ever, but virtually nothing once they went to the water," Fedak said. He tried attaching small radio transmitters, with mixed results. Then in 1990, on South Georgia Island, in the southern Atlantic Ocean, Fedak affixed an experimental satellite tag to a southern elephant seal that his team had dubbed Mrs. Nasty. She was "a real stroppy animal," Fedak recalled.

Days later, a terse message came over the marine band radio from an Antarctic base that had a relay back to Cambridge: Mrs. Nasty was on the move. All Fedak knew then was that she had gone south. It was working! He was beside himself.

The volume of data was overwhelming. The researchers were still crunching data using a mainframe computer. Back in Cambridge, Fedak's team at the British Antarctic Survey ran rolls of paper down the hallway—printouts of the seal's diving depths, locations, and times—while holding up a chart of the ocean's depths alongside them. They worked out that Mrs. Nasty had made straight for a certain notch in the Antarctic Peninsula where deep circumpolar water flowed. "It was like having magic binoculars," Fedak said.

Not long after, Fedak began collaborating with the oceanographer Ole Anders Nøst at the Norwegian Polar Institute. Fedak wanted to measure ocean temperature. Nøst wanted to learn more about the Arctic Ocean's structure near Svalbard. The area was a known beluga haunt, so the Sea Mammal Research Unit outfitted whales with satellite tags that could record their GPS position depth, temperature, and salinity. Under the ice, the belugas found an influx of warm North Atlantic water that nobody had suspected was there.

Not every potential collaboration went so smoothly. At a scientists' meeting in the 1990s, Fedak heard oceanographers proposing to deploy data-collecting buoys out in the notoriously rough waters of the northwest Atlantic. Fedak suggested a different approach: Tag the hooded seals in those areas. "I got laughed at," he said. "Some old oceanographer in the back of the room stood up and said, 'You just want our money!'"

The oceanographer wasn't wrong about the money part. Oceanographers were far more flush with research dollars than biologists, since their work was so essential for the military, shipping, and other practical applications. But the potential was real.

"We wanted to understand the animals and the environmental conditions they needed," Fedak said. "And the oceanographers were desperate for the data."

THE TAGS THESE DAYS! They've improved by leaps and bounds, into feats of miniaturization, energy efficiency, information compression, and hardiness in the face of a powerful and indifferent ocean. X1 left Año Nuevo with an accelerometer on her jaw that recorded every time she grabbed a fish. Other seals travel with a miniature head-mounted camera with an LED infrared flash, programmed to record one minute of video when they open their jaws at least 1,300 feet down. That's how I saw a picture of a fish in its last moments before being eaten. It looked startled, as fish do. Many scientists are jazzed about new tags that will measure dissolved oxygen and chlorophyll, and help track the biological productivity of the water that animals like X1 move through.

X1's main tag, looking like a jaunty hat with a stubby antenna, is the core AniBOS model, collecting salinity, temperature, and depth, plus time and location. It's known as a CTD tag because salinity is measured via conductivity.

Salinity, temperature, oxygen: This is the code in which the movement of the world's waters is recorded. Oceans are composed of distinct masses of water, spread across thousands of miles, that are as unwilling to mix with one another as oil and water because their densities are so different. With salinity, temperature, and oxygen, oceanographers can track these masses across the planet. The movements of these waters—how they transport heat and nutrients and store carbon—are crucial to understanding the fates of all creatures, including humans. They will help predict whether people and sea creatures alike can find food or protection from increasingly extreme weather. They can help calculate how much the sea will rise and how it will threaten our communities and coastlines.

RACHEL HOLSER DIDN'T SET OUT to study the marine heat wave that took over the North Pacific Ocean in 2014 and 2015. She didn't need to. The Blob swallowed her dissertation on its own.

Years before, as a seabird-project field tech fresh out of college on the Pribilof Islands, in the middle of the Bering Sea, she witnessed huge, unexplained reproductive failures among the seabirds and fur seals. Holser, who had studied systems ecology, thought that surely the ocean must be driving this shortage of descendants. That animated her to get an oceanography degree before going back to biology.

Costa, who had been tagging seals for three decades, brought on Holser to collect data on the ocean as well as the seals in it. In fall 2014, Holser tagged her first seals. They were headed for the North Pacific, a stretch of ocean that is very hard to measure. The data that rolled in revealed that the water around the seals was 6 degrees warmer than it was supposed to be.

"I was looking at these temperatures like, 'This is crazy—the Pacific should not be 20 degrees [Celsius] at the surface,'" said Holser, who is now an assistant researcher in the Costa lab.

The seals had swum straight into what turned out to be the Blob.

At the time, marine heat waves had been a focus of dedicated research for only about a decade, but it was clear that this one was a record-breaker: the biggest, the second hottest, and the longest lasting. The mystery was where it had come from, and how far down into the ocean it reached. Measurements from ships showed higher temperatures as far down as 600 feet. But it took the elephant seals to see the full extent of the weirdness.

On their deepest dives, 2,000 or 3,000 feet, tagged seals still recorded intensely anomalous warm water. In that twilight zone, a quarter of a degree Celsius is an ocean of difference, since the physiologies of animals living there are exquisitely calibrated to stable temperatures. The arrangement of these anomalous patches hinted at the Blob's origin.

"That water would have had to travel to create some of those deep, warm anomalies, because you don't get warm down at 800 meters [2,625 feet] as a result of the sun shining on top of the ocean," Holser said.

The seals that traveled through the Blob were changed by the experience. Some of them got astonishingly fat. Their new physiques pointed to a changed diet, like fattier fish. The seals that returned were healthy, but more seals failed to come back than had in previous years.

"If we weren't studying elephant seals, we might not have had any indication that there was some sort of shift in the twilight zone in those two years," Holser said.

SEAL DATA ARE PARTICULARLY VALUABLE because the ocean is not all equally interesting. Researchers told me this many times, and I can confirm it even as a surface dweller. When I sailed a small boat across the Tasman Sea between Australia and New Caledonia, a 12-day trip, I hardly saw a bird for a week in the middle, and my fishing line caught nothing but a single, spooky tentacle one night. As soon as I crossed into the waters near New Caledonia, the seas and skies came alive with animals. That's how it is: You get barren stretches and lively patches, and the latter are often where two water masses collide, mixing nutrients in the water column. The phytoplankton feed on the nutrients, the zooplankton feed on them, and everything else up to albatrosses and seals shows up for the buffet.

"Those areas of high physical and biological activity—that the animals will go to because it's of interest to them—are in fact the kinds of places we want to sample," said AniBOS's McMahon. Seals head right to them, like truffle pigs in a hazelnut grove.

Beltran is using seal data to find these biological hot spots. Seals are sentinels, she said, because they collect such a vast amount of environmental information not only via the tags but also within their own bodies. A seal may linger in water that looks empty and still manage to pack on 600 pounds in seven months. But first she has to pick the right seals to tag. This begins with the annual Weigh the Weanlings extravaganza, which Beltran was running on the day I visited Año Nuevo.

As Holser hunched over X1, Beltran and an undergraduate volunteer, Wade Matern, deftly wrestled a weanling into a cylindrical tarp bag. This took several minutes, not because of any real struggle from the seal but because a second weanling took an interest in the situation. Beltran patted it gently in the direction it should go. "Get! Out! Of! Here!" she told the animal. It stayed.

Weanling secured, two other crew members centered a tall tripod over what was now a well-wrapped seal burrito. Beltran winched it up just a few inches, which took a lot of work. A nice fat pup, at 317 pounds (well over the average of 260). Beltran spread the weanling's tail flippers so that a helper could tag it; at this, the weanling lurched briefly. It was released from the bag, and it settled back down in its sunbeam.

Out of the 300 weanlings that were weighed that year, about 40 would likely make it to age four. The fatter they got while still nursing, the better shot they had to live through the vulnerable postweaning period before they knew how to swim or fish. Each time they returned to this beach, many would be noted, weighed, and tracked.

It's rare to have such a long-term data set in biology. So much of it is snapshots in time. "The more you know, the more valuable each additional piece of information becomes," said Patrick Robinson, director of the Año Nuevo Reserve and another Costa lab graduate. And, he added, the more you know about the biology of a species, the more you know how biology and oceanography are linked."

IF YOU WANT TO UNDERSTAND the planet's climate, go to the Southern Ocean, where the Antarctic Circumpolar Current—the mightiest of all currents—makes an endless rushing gyre around the polar continent under a constant march of storms. The current's power has fascinated AniBOS's Roquet since he was an engineer at the French research station on the Kerguelen Islands, south of Madagascar, where the Indian and the Southern Oceans meet. "Things are changing slower in the Southern Ocean," Roquet said, "but it's a slow and powerful change. Once they start to take place, it's very hard to stop."

We mostly hear about Antarctica's ice when it melts, but it's in a constant state of flux, forming all winter and melting in the summer. As ocean water freezes, it discards its salt, which drops like an underwater waterfall, carrying oxygen along with it down into the deep ocean, where it becomes what's known as Antarctic bottom water. This process drives currents around the planet.

"Understanding where and how much Antarctic bottom water is formed each year, and whether that's changing over time, is central to us understanding the global movement of water and heat and oxygen and carbon dioxide," McMahon said. "I can't overstress the importance of Antarctic bottom water to understanding global climate."

He has come to think of it as "the beating heart of the planet."

Global warming is really ocean warming: 90 percent of the heat we have generated over the past half century has been absorbed by the ocean. The surface is warming, but what is happening lower down in the water column is still mysterious. This ongoing exchange between ocean and ice, and particularly the dense, salty water that forms under ice as it freezes, has massive implications. "If the dense water formation is strong, we have a strong, regular heartbeat that can pump water around the oceans," McMahon said. "It can exchange heat, oxygen, all of those things, exactly the same as what your heart does to your blood around your body. If that process of dense water breaks down, we break down the currents."

The heart exam results are not in yet. Oceanography requires precise instrumentation and very long-term observations. For 30 years, oceanographers knew, based on their calculations, that there had to be more Antarctic bottom water than what they had charted, but its location was a mystery. It was as if the cardiologists did not know quite where the aorta was.

Then, about 10 years ago, some tagged seals swam into a polynya, which is a gap in the sea ice made by wind. It was off Cape Darnley, an Antarctic peninsula south of the Kerguelen Islands, and it was the missing aorta. Roquet and his colleagues calculated that this polynya was producing around one-tenth of all the Antarctic bottom water. The news of its location came from the southern elephant seals, the queens of AniBOS. They had gone right to the heart of it.

TEN WEEKS AFTER X1 LEFT, Holser told me, she was back on the beach. Her tags recorded that she had spent 75 days chowing down on the all-you-can-eat buffet that is the seabed off Oregon's coast. X1 had returned to Año Nuevo at 1,100 pounds, or 40 percent fatter—"pretty dang good," Holser said. Older females are often better at foraging. By going to the coast instead of out to sea, X1 had acted, unusually, more like the males: Seabed foraging is more of a gamble but can pay off with bigger fish.

The beach was for now occupied by an all-female crowd, which was way more relaxed because nobody was trying to hump them. X1 could molt in peace. In her 13 years, ever since she was a weanling, every life milestone, every pup, every encounter with a researcher, had been entered on a spreadsheet, some 250 observations. The researchers would still keep tabs on her and her descendants. But this last trip had been her final deployment. Three is the max, for ethical reasons. In another six weeks, she would leave for her annual long trip of about eight months. Whatever hot spot or Blob or anomaly she discovered, she would witness it alone.

This article appeared in the Fall 2022 quarterly edition with the headline "Swimming in Data."