Thorrold of the Woods Hole Oceanographic Institution, Massachusetts, has signed up to be an ocean-going researcher assistant for whale sharks. They are fitted with electronic tags that include a variety of sensors, tracking devices, and sometimes tiny cameras. This allows them to gather information that human researchers cannot. They have demonstrated that large marine predators can dive deep, and have made remarkable journeys across vast oceans.

Many animals are at risk from fatal decompression sickness by plunging to hundreds, sometimes even thousands, of meters below the surface.

Deep-diving species have developed a variety of physiological and anatomical adaptations to function at these depths. These include thick, insulating, blood vessels that can be transformed into heat exchange systems, collapsible lung and oxygen-storing muscle, as well as ultra-sensitive eyes. What is the secret to these incredible diving abilities?

Most biologists will answer the question with one simple answer: Food. But it's been difficult to prove. There is enough evidence from decades of tagging to prove that top predators dive deep for prey. Only one species has yet to be seen in action. A pioneering series featuring the northern elephant seal (Mirounga Angustirostris), its own whiskers, and a supporting cast including deep-sea fishes and squid has made it a star.

Thorrold says that food is not the only draw to the deep sea. He co-authored an article on the motivation of diving predators, which was published in the 2022 Annual Review of Marine Science. Divers and divers behave differently: Some animals dive frequently, while others only occasionally. The mesopelagic is a deep-water zone that extends to 200-1000 meters. Some dive deeper than this, while others go much deeper. Divers may also be able to identify multiple functions by the way they look. For example, a quick, steep descent and equally steep ascent suggests a different purpose than a slow, long, flat-bottomed dive. Thorrold says that if the same person does different types or dives at different times, then it's clear evidence they have different purposes.

There are many possibilities for these purposes. Deep, darkly lit waters could be used to provide shelter from predators; cool off an overheating body; or even to provide navigational cues to those who are able to spot them. Thorrold says that all these possibilities are possible. "The fact that we can't rule them out reveals how mysterious large pelagic animal groups are to us."

Nearly every ocean-going vertebrate has learned to dive deep. It's done by big bony fish like swordfish and tuna. It's possible for rays and cartilaginous sharks to do it. Air-breathing animals such as penguins and sea turtles, toothed whales, seals, and penguins can also do it. They all reach incredible depths in a single breath.

Many dive to the twilight area, which is where the dimming light from above quickly fades to nothing. Some dive into the darkness of the midnight zone (the bathypelagic region between 1,000 and 4000 meters). Cuvier's beaked Whale is currently the record holder. In 2014, one tag whale reached 2,992 metres off the coast of Southern California. A whale shark, which reached 1,928 metres in the Gulf of Mexico in 2010, holds the record for fish.

In the past, biologists would not have imagined that deep waters could provide a high-predator predator. Naturalists believed that no one lived below 500m in the 19th century. But Navy sonar operators discovered the deep scattering zone, where their sonar bounced off a multitude of mesopelagic creatures. As tiny invertebrates and fish moved to the surface at night to feed, the food-packed layer moved upward and downward.

The ocean's twilight area turned out to be an unexpectedly rich larder. It was home to bizarre and delicious gelatinous creatures, muscle squid and the highly nutritious lanternfish. Based on net surveys, fisheries scientists in 1980 estimated that the mesopelagic fish biomass was one billion metric tons. Acoustic surveys were used to estimate the figure at seven to ten times higher in 2014.

Although there is not yet a global estimate of how many animals live in the midnight zone's dark, chilling waters, a Mid-Atlantic Ridge study revealed that there are more potential prey. Thorrold says that diving to forage makes sense when deep water is the place where biomass is.

However, until very recently, the only evidence supporting foraging was circumstantial. The stomachs of blue sharks, blue sharks and swordfish contained mesopelagic fish, squid, and crustaceans. Sperm whale stomachs were home to the indigestible beaks deep-sea shrimps, including the Architeuthis, which was a large squid. According to tagging studies, predators and prey are always in the same spot at the same moment. The researchers have shown that large fish and mammal species regularly dive into the deep scattering layer. They also dive deeper when the potential prey has moved further into the darkness. Tuna and swordfish tags track the daily migration of potential prey.

Biologists have been able to build a more complete picture of the deep-sea animals' activities through the use of sophisticated tags. They are attached to flippers, jaws, fins, and flippers and can store and retrieve a large variety of data over several months. Researchers can reconstruct the movements of divers through the water by using tags that contain sensors for temperature, pressure and light. Innovative extras have been developed over the past few years. These include accelerometers that record the turning and twisting of a head, sound-detecting hydrophones that detect jaw movement, and even a smart video camera which records only a fraction of what is recorded.

It is difficult to get these tags on top predators. Thorrold says that whale sharks are rare and difficult to find, but they have been tagged in a number of cases. For a different reason, fearsome sharks such as the great white can be difficult to capture. You cannot dive for free. If you wish to tag the dorsal fin of the shark, you will need to use a ship equipped with hydraulic platforms to lift it aboard. This allows you to safely carry out the operation.

Swordfish can be difficult to tag. Thorrold's coauthor Peter Gaube is an oceanographer from the University of Washington. He can confirm that they are difficult to catch, unpredictable, and dangerous. You must hold the fish alongside the boat to repair the tags.

The best evidence of foraging comes from surprising places, such as the Chilean Devil Ray (Mobula tarapacana), which is a mysterious and huge fish that has a "wing span" of nearly 4 meters. The majority of Chilean devil Ray sightings are from the surface, where they bask in the sun. This habit led to the assumption they prefer living near the surface.

Thorrold and his colleagues went on two expeditions to the Azores to tag devil rays. They spend a few months circling the Princess Alice seamount every year. The team tagged four rays in 2011, and 11 more in 2012. The tags kept track of the movements of the rays for up to five years before sending their data back to Woods Hole.

These results were astonishing: The devil rays traveled thousands of kilometers at an incredible speed and often plunged more than 1,000 meters. The 1,896-meter deepest dive was recorded. These sun-soaked, surface-dwelling Rays are nothing but. They are some of the deepest-diving fish in the ocean, and all evidence points to food being the main attraction.

The unusual stepwise profile of most devil ray dives was what made them so popular. Thorrold says, "They dive deep, then come up and level again. Sonar shows that they seem to stop at the areas where there are dense, thin layers of prey. Although we have not been able to see their activity, this strong evidence suggests that they are actively foraging.

This would explain why Chilean Devil Rays have well-developed blood vessels within their brain cavity. It functions similarly to that of deep-diving sharks. It was always a mystery to biologists why fish living in the upper ocean's sunlit waters would require one. Thorrold says, "This is more proof that devil Rays forage deep and so need to keep the brain and sensory system active in cold."

Thorrold suggests that you should warm up before and during deep dives.

Similar to devil rays and tag-toting sharks, they have provided fascinating glimpses into their behavior which lends more weight the notion that they hunt deep in deep water. While most of our knowledge about sharks is based in coastal waters, many of them migrate across open oceans. If you are not close to seal-studded coastlines, your prey will be scattered and patchy. How do large sharks manage to get enough food?

Recent research shows that sharks use smart strategies to get to the ocean's largest buffet. Two great white sharks, and 15 blue sharks were able to show that they use eddies, which are swirling masses that break from the Gulf Stream. The Gulf Stream's northern edge traps warmer water, while eddies that form from the southern edge transport cool water southward. Both blue and white sharks preferred warm-hearted eddies.

Acoustic surveys have revealed that warm eddies are home to a greater number of mesopelagic prey. Sharks can also forage deeper and longer in anomalously warm water that extends hundreds of metres. Gaube, coauthor of the research, said that warm eddies could allow sharks to access deeper food sources that might otherwise be unaccessible.

Animals with tags that have extra bells or whistles are the closest thing to proof that marine predators developed extreme diving skills in order to exploit rich, but otherwise inaccessible sources of food.

The sound recorder was used in the case of Globicephala macrorhynchus, a short-finned pilot whale. While they hunt, pilot whales emit a series clicks to listen for echos from prey. The clicks become so loud and rapid that they resemble a buzz as they get closer to their target. Natacha Aguilar de Soto is a marine biologist from the University of La Laguna in Tenerife. She spied on 23 local pilot whales as they dived and fitted them with sound-recording tags.

The tagged whales dived deep to reach a maximum depth of 1,019 metres, clicking as they went. The clicks became buzzes just before the depth of the dive. This is a sign that the whale was about ready to attack. Aguilar De Soto believes that a whale will make a last, fast dash to escape prey. This is what happens when it dives very deep.

Although the sounds of hunt are convincing, it is not enough to prove their existence. Thorrold says, "We must see what these predators do." Biologists will have to be content with what little film was taken by the northern elephant sealing for now.

Research assistants for female northern elephant seals are the colony of Ano Nuevo State Park's colony, north of Santa Cruz. Since more than 50 years, biologists from the University of California, Santa Cruz have run a research program. Ano Nuevo elephant sealings have the advantage that they are easily accessible. They travel on land in winter to mat and pup, and then again in spring and summer to molt.

The males stay in the coast waters while the females travel thousands of kilometers across the Pacific, diving as they go. It is much easier to attach tags and retrieve them later than for great white sharks. Expert elephant seal wranglers are available to assist. Taiki Adachi, a Japanese biologist who has been working with seals for over ten years and is currently based in UCSC, says that it can be dangerous. They are aggressive and large, which makes them especially frightening during breeding season when mothers must protect their pups, while the males defend their harem.

Adachi reported recently that migrating female Elephant Seals dive almost continuously for up to 20 hours per day. He says that they typically dive to 400-600 meters. This is the depth at which small fish, especially oil-rich lanternfish, are abundant. They can dive to 800m or deeper. The highest recorded distance for this species is 1,735 metres.

Yasuhiko Naido, Adachi's Japanese colleague, created a jaw-motion recorder to log seals' attempts at grabbing prey from the sea. The device was attached to the seal's lower jaw and logged between 1,000 and 2,000 attempts per day. Naito's other innovations were the clincher. A smart video tag is attached to the seals' jaws or heads. The combination of depth and movements typical of a strike triggers the camera and lights, making the most of the limited battery power.

One individual elephant seal took the first selfies of an elephant seal, and published them in 2017. It was seen trying to catch fish 800m below. These 21 fuzzy images revealed parts of what was later identified as deep-sea, large-sized ragfish. The team was able to capture almost 700 attacks with the help of additional camera-carrying seals. They had 48 hours worth of footage, ranging from 240m to over 1,000m deep. It was possible to identify the prey type and sometimes even the species thanks to the excellent quality of the footage. These included the little lanternfish, ragfish, and a type hake. There were also half a dozen squid including cockeyed and glass squid.

Divers do not just dive for food. There is also evidence that extreme dives serve other purposes. Escape from larger predators is a strong contender.

Yellowfin tuna (Thunnus Albacares) spends most of its time in the ocean's 200-meter upper zone. Tim Lam, a fisheries biologist at the University of Massachusetts Boston reported in 2020 that six of the 17 tuna he had tagged off the coast of Hawaii were bitten by a predator. Lam suggests that four of the 17 tuna he tagged in the waters off Hawaii dived to depths of around 1,000m, three of which were at about 1,000m. They then lost their tags, possibly due to frantic escape attempts. The fifth tuna plunged abruptly from 134 meters down to 1,592 meters, which Lam interprets as an attempt to escape an enemy. It returned to the surface with the jitters and spent the entire day close to the surface before returning to normal.

Then there was the one that couldn't be found: Data from its tag indicated that it had reached depths of 326 metres. After that, everything went dark and temperatures rose. This could have been because it was in the stomach of a false killer whale, short-finned, or pilot whale.

The dimly lit depths are also a good place for elephant seals to hide from their enemies. According to Roxanne Beltran (physiological ecologist), the most common cause of death is shark or killer whale predation while at sea. Roxanne Beltran works on the Ano Nuevo Elephant Seal Program. "But, we see many seals coming to shore with fresh shark bites or healing ones. So clearly it is possible to evade their predators."

The other is to go down. A test run of a new tag, more than a decade old, gave us an early indication that seals go that way. Bioacoustician Selene Fregosi from Oregon State University provided a prototype tag to play back recorded sounds to young elephant seals at Ano Nuevo as part of her research into the effects of underwater noise. It was intended to test how seals react to brief bursts with different noises. The playlist contained whistles and clicks from sperm and killer whales, which were used to echolocate and send elephant seals into deep dives. It would accelerate if a seal was already diving; if it was returning to the surface, it would turn tail and dive deeper. In one instance, it almost doubled its original depth.

Beltran and his colleagues last year reported that elephant seals do not just flee into darkness; they also rest in the dark. In brightly lit areas of the upper ocean where killer whales and sharks are frequent, elephant seals are more at risk. They only have a few minutes to rest after foraging and can drift effortlessly for 10-15 to 20 minutes. Beltran discovered that their preference is to rest several hundred metres below the surface. They are more fit and fatter than ever, so they will go deeper to find greater safety.

Are there other reasons to dive into the deep?

Navigation is highly probable. Nearly all large marine predators migrate over long distances at one time or another in their lives. Some animals, including turtles and sharks, are capable of sensing magnetic fields and anomalies in the earth's magnetic field. Thorrold says that animals that are able to sense these cues may dive deeper, where signals are stronger. Long migrations are the only time leatherback turtles will make extreme dives. This suggests that they may be checking if they're on the right path. The Gulf of California's Hammerhead sharks are believed to use magnetic "landscape" to navigate to seamounts.

One species appears to be able to dive to cool down. Atlantic bluefin tuna spends months in cold waters each year and has developed a highly efficient way to keep their body heat. However, they spawn in subtropical Gulf of Mexico where this is a handicap. To avoid overheating, tuna dive below 500m as they enter and exit the Gulf. They also stay below 200m while they spawn.

The deep sea may be a great place to communicate. The zone stretches from a depth between a few hundred and more than a thousand meters. Sound travels farther, making it an ideal place for communication over long distances. Blue and fin whales can be heard up to 1,700 km away when they are in the zone. However, no one knows whether they visit this area for specific purposes.

Thorrold says, "There is so much we don’t know, even with all the technology that’s been available over the past 20 years." There are many future tech goals. Thorrold and his coworkers are currently testing a prototype tag which can pinpoint the position of an animal in the water column. They would love tags that only return the relevant information from the large number of data collected over several months at sea.

Small, intelligent cameras are at the top of this list. Why can't elephant seals make great movies? Thorrold says, "We need fish-cams with better resolution that can be mounted on tunas, sharks, and swordfish." They need to be small but highly-reliable, capable of operating in low light levels and logging data for long periods while they travel the open ocean.

Seeing is believing, in a nutshell. A great white shark's home movie is something that everyone would love to see.