In dark water diving, the ocean floor is invisible, says Jialing Cai. You float in an infinite black void.
“There is no reference point that allows you to know where you are in this three-dimensional space,” explains the specialized underwater photographer.
“If you are not aware of how deep you are, you could start to sink deeper and deeper.”
However, he says that during his first night dive, his excitement overcame his fear.
Cai soon discovered that this black “empty space” was, in fact, teeming with life: from baby octopuses, jellyfish and crabs, to juvenile fish and all kinds of copepods, or “insects of the sea.”
Cai, originally from Chongqing, China, is an award-winning darkwater photographer.
In 2023, she was named Ocean Photographer of the Year by the magazine Oceanographic for its image of a paper nautilus, a rare octopus with a paper-thin shell.
Then in 2025, Cai won the Female Fifty Fathoms Awards from Oceanographic for his collection of photographs that captured some of the ocean’s most charismatic visitors from the depths.
However, Cai doesn’t need to dive very deep to capture these creatures from the ocean floor.
Simply wait for them to emerge from the depths at night, when they rise to the surface to feed, in what constitutes the largest mass migration of animals on Earth.
Very little is known about the true nature of life in the middle layers of the ocean.
Therefore, specimens are collected with nets. Once removed from their natural environment, their lifeless bodies—often damaged—are preserved in jars.
But these stunning images reveal the astonishing daily life of the ocean’s interior, says Jon Copley, professor of ocean exploration at the University of Southampton in the United Kingdom.
“Having photographic evidence of these animals is important because they are rarely seen in their natural habitat. Traditionally, specimens of mid-ocean animals like these are collected with nets, and these do not preserve the complex structures that can be seen in a photo.”
In December 2018, at just 19 years old, Cai embarked on his first dark water dive in Batangas Bay, Philippines.
In the water, the color disappears very quickly as you go down, Cai says.
“You have to get very close to the subject if you want to capture a good image. You have to stay relatively still, even when they move. And sometimes you find a subject, you get close to it, and then it starts swimming. Then you have to chase it.”
Fully focused on photographing her first deep-ocean species, Cai swam away from the boat and her dive guide, who was in the water with her.
That’s when he captured the image of a wunderpus octopus (seen above).
As an adult, this wunderpus will have zebra-like stripes, but in its juvenile stage, Cai says, its color cells have not yet developed.
“Almost transparent, it can camouflage itself in dark, empty space.”
Cai used three bright lights to capture this image, which also attracted all kinds of zooplankton—shrimp larvae, crabs, and worms—to join in the fun. “There is an enormous diversity of life here. It’s like a competition.”
Suddenly, however, Cai realized that he had swum so far from the ship that he could no longer see its lights. Alone in the dark, she turned around in the water and saw nothing.
“I turned off all my lights: the camera lights and the hunting lights.” He hoped this would allow him to detect the faint light of the ship in the distance.
Still, there was nothing but the darkness of the night ocean.
Cai surfaced and turned on his lights. That’s when the dive boat crew saw her and, after a few minutes that seemed like an eternity, they pulled her to safety.
An ordinary diver
When Cai was studying marine biology at the University of Virginia, in the United States, he thought that exploring the deep sea would be as ambitious as going to space.
Then one day, his teacher explained the phenomenon of diurnal vertical migration (DVM) to him.
Every night, he explained, billions of tiny zooplankton rise from the deep sea to feed under the relative safety of darkness.
They swim hundreds of meters into shallow water, and some even reach the surface.
During this nocturnal migration, the ocean vibrates with an “evening chorus.” It is the murmur of countless fish, shrimp, jellyfish and squid heading towards the surface.
Later, just before dawn, they return to the abyss.
“Diurnal vertical migration occurs in all marine and freshwater environments on the planet,” explains Laura Hobbs, professor of Arctic marine sciences at the Scottish Marine Science Association.
“This is the largest migration in terms of biomass on the planet, and it happens every day.”
“It hit me like a bolt of lightning,” Cai says. “I couldn’t help but interrupt the teacher. I asked her, ‘Are you saying that I don’t actually have to dive into the deep sea to see the creatures for myself? Can I just be an ordinary diver?’
“I realized that I could catch a glimpse of a world that my mortal body would never allow me to reach. I just have to throw myself into the open ocean at night, and the depths of the sea will come to me!” he thought.
So Cai learned to dive and use a camera.
Today, he specializes in photographing zooplankton in the darkness of the open ocean at night.
Without diving deeper than 30 meters, Cai photographed these ocean floor animals swimming to meet her.
The dance of the immortal jellyfish
This immortal adult jellyfish “looked like a light bulb in the ocean night,” says Cai.
The immortal jellyfish gets its name from its ability to “revert its state to the brood stage, the polyp stage,” upon detecting danger, such as the presence of pathogens in the water, he explains.
“It can restart its life cycle over and over again.” In theory, immortal jellyfish can live forever.
The jellyfish was frightened by Cai’s bright light and immediately retracted its tentacles when she approached.
So she switched to a red light, using a white flash to capture the image in an instant.
Immediately, the jellyfish began to spread its tentacles, dancing like a swirling tornado.
“Sometimes you have to use these kinds of tricks to get sea creatures to show us how they really behave,” Cai says.
Borrowed poison
The group that participates in the daily vertical migration is a multitude in which predators and prey mix.
“These little creatures are the bottom of the food chain,” says Hobbs, “so they’re really important to everyone from fish to whales.”
To survive and avoid being eaten by larger predators—or each other—many of these animals have developed ingenious defense strategies to protect themselves.
This young fish has captured a jellyfish with its mouth, taking advantage of the toxins in its tentacles.
“This is very typical behavior in these young fish,” says Cai. “They take advantage of the jellyfish’s poisonous tentacles. Every time I tried to get close to the fish, it immediately directed its chemical shield at me.”
Fascinating vision
“Here you can see a juvenile horse mackerel, a species of fish from the carangid family, perched on the bell of a jellyfish,” explains Cai.
Inside the jellyfish, you can see a baby octopus being digested.
“Its soft, blood-stained eyes are visible inside the gelatinous stomach. To the baby octopus, this jellyfish is merciless.”
For many small zooplankton, Cai says, the jellyfish’s gut becomes their final resting place. For others, such as the trevally fish, the jellyfish offers refuge.
Cai’s images, Hobbs says, offer a fascinating glimpse into the diversity of life at this scale in the ocean.
“These huge migrations occur as part of a complex three-dimensional environment, where tiny zooplankton move to the surface to feed while at the same time facing a variety of predators.
“It’s amazing to think that each of these species follows its own strategy, shaped by its preferred prey, its potential predators and its reproductive needs.”
Refuge in the trash
Here, “two little fish, each no bigger than my thumbnail, sail across the open ocean at night under the fragile shelter of a drifting piece of candy wrapper,” Cai says.
“An involuntary refuge in a world increasingly marked by human waste.”
The twilight zone—or mesopelagic zone—is a vast layer of water that extends across the entire planet.
It reaches from 200 meters of darkness under the waves to total darkness at 1,000 meters deep.
This part of the ocean is home to the vast majority of marine life—with more fish than the rest of the ocean combined—and is important for both ocean ecosystems and the global carbon cycle.
There are several ways in which human activity negatively affects the twilight zone, Copley says.
“These are depths where some species of fish are caught. Then, of course, the other big impact on the entire deep ocean is climate change. This affects the twilight zone and what lives there. Predictions indicate that the abundance of life there will decrease.”
This area needs urgent protection, Copley says, as research shows that climate change will reduce life there by up to 40% by the end of the century.
Metamorphosis
Some of these tiny creatures undertake epic journeys before reaching our shores.
Crabs, for example, are often considered inhabitants of the seabed. However, like most marine animals that we find in coastal areas, they go through a planktonic stage, explains Cai.
In their larval phase, they drift in the water column, going through multiple stages of metamorphosis, before finally settling in their definitive habitat.
“This image captures a juvenile crab in the process of molting, shedding its old exoskeleton almost as if taking off a sweater,” details Cai.
“If you look closely, you can see subtle differences in shape and structure between the transparent molt and the newly emerged body of the crab.”
So, the Next time you see a common beach crab scampering around the environment, consider that it could well have traveled more than 100 kilometers into the depths of the ocean, surviving against all odds.
*This article was published on BBC Future.Click here to scrutinize the common version (in English)
click hereto scrutinize more stories from BBC Files World.
Subscribe hereto our new newsletter to receive a selection of our best content of the week every Friday.
You can also follow us on YouTube, instagram, TikTok, x, Facebookand in our new whatsapp channel.
And remember that you can receive notifications in our app. Download the latest version and activate them.
- Spectacular images showing unique marine creatures and corals on Caribbean islands
- The ghost shark, the carnivorous sponge and 1,100 other marine species discovered in just one year
