'You look up and see light coming through': The divers venturing under the ice in the name of science

There is a 70cm-thick (28in) layer of ice capping the surface of this lake, in a remote corner of Lapland, northern Finland. Gathered around a hole cut into the ice is a group of around 20 people, peering down into the inky depths with some trepidation. The seemingly lifeless water below the ice has a temperature only slightly above 0C (32F). Some of them are about to jump down there to venture under the ice. 

Sophie Kalkowski-Pope is one of the divers preparing to visit this strange, upside-down world where she will swim below a ceiling of smooth ice. The marine biology graduate from the University of Queensland, Australia, is part of an ice-diving training party that has gathered here. She is wearing a dry suit and anticipating the initial cold shock when that frigid water will hit the exposed skin on her face.

It's so numbing in these Arctic waters that, even with thermal clothing and special insulating gloves, divers find it hard to use their hands after just 30 minutes underwater. There are other dangers, too, so strict safety protocols are in place. Divers are tethered to the surface using a safety rope, with a handler on the surface communicating with the diver via rope signals. One tug for "ok", two tugs for "stop", three for "come back".

A standby diver waits nearby in full gear, ready to enter the water if there's an emergency. And there are two holes cut in the ice next to each other so there are two exit points.

The lines of rope linking divers back to the surface could become entangled in submerged branches or logs, other debris, or even the diver's own equipment such as fins or tanks.

The training here on a frozen lake is practice for work that will be done out on the sea ice of the Arctic and Antarctic, where there are added dangers – large seals sometimes gather at the dive holes, preventing divers from leaving the water.

With one final check of her equipment, Kalkowski-Pope puts her breathing apparatus in her mouth – and flings herself in.

Those twinkling lights are like beacons that guide the divers back to the surface

Today, ice is still a key feature of the Arctic but it is becoming less and less common. Because of climate change, the Arctic is warming four times faster than the rest of the world. Satellite data reveals the area of sea ice covering the Arctic Ocean has declined by around 13.2% per decade, on average, since monitoring began in 1979.

Scientists, driven to understand the changes unfolding here, are going to extremes to carry out their fieldwork. Some are learning ice diving techniques, so that they may observe underwater Arctic ecosystems and document the hastening retreat of the ice. Their short, and sometimes dangerous, expeditions are revealing the secrets of a rapidly thawing world. 

"Once you get in the water, you realise what a beautiful environment you're in and you calm down a bit," says Kalkowski-Pope as she describes the natural trepidation that people have ahead of an ice dive. 

She has come to Kilpisjärvi in Lapland, over 248 miles (400km) north of the Arctic Circle, to join other divers from all over the world on an ice diving training course. The trainees plod gingerly around two rectangular entry holes that they've cut in the lake's ice. 

It may be the middle of March but it still feels very much like winter here. The lake is frozen over and surrounded by Finland's highest fells, still draped in thick snow.

Perry Brandes, a commercial diver from Florida, where he is used to a far warmer climate, has just completed his first ice dive. "It's very peaceful," he says. "You look up and see light coming through. It's like looking at a city from afar." This, he explains, is the sun peeking through holes in the ice above. Those twinkling lights are like beacons that guide the divers back to the surface. (You can read more about what it is like under the ice in Antarctica in Katherine Latham's fascinating article about this upside down ice-scape.)

Kalkowski-Pope and Brandes are two of 12 participants on the week-long polar research diving course, which has been organised by the University of Helsinki.

All of those taking part are already seasoned divers who have honed their skills in temperate and tropical waters. This is the next level. Should they master ice diving here, it could enable them to join scientific diving expeditions in the Arctic or Antarctic. 

"There's probably only a few hundred people in the world who can do polar base diving work at the moment," says Edd Stockdale, coordinator of the Finnish Scientific Diving Academy, who leads the course. "The polar areas are melting. We need scientists who are able to monitor what's happening," he explains.

Scientific research in the polar regions is critically important for climate change monitoring. The melting of polar ice contributes to rising sea levels around the world. By studying changes in sea ice, scientists are able to gain a better understanding of how quickly climate change is unfolding.

Ice diving, in particular, allows researchers to collect first-hand data on ice thickness, density and movement as well as water temperatures and salinity. The polar regions also have unique flora and fauna adapted to living in extremely cold conditions, and some of these organisms are visible during ice dives.

In 2017, Alf Norkko, a professor of marine research at the University of Helsinki, and his team discovered big changes on the seafloor under Antarctic sea ice since their previous diving expedition in the same area in 2009.  

"There was a remarkable increase in the abundance of life," he says. Levels of chlorophyll and other plant compounds deposited in the sediment on the sea floor had risen dramatically, indicating that the amount of plankton and algae in the water had increased. "In just a few years, the sea ice had got thinner, which allowed more light to get through so there was more food supply for starfish, worms, sponges and sea spiders on the seafloor."

A recently published study of kelp forests off the coast of the Arctic archipelago of Svalbard saw divers make repeated trips to the same site over a 25 year period to examine how warming temperatures were affecting these important ecosystem by changing the mix of seaweed growing there.

Data collected by research divers from the British Antarctic Survey's Rothera Research Station also recently helped to show that the coastal seabed off the West Antarctic Peninsula is more frequently being struck by icebergs due to reductions in sea ice. These collisions cause catastrophic damage, killing almost everything in their path and scouring enormous scars along the seabed.

Norkko adds that it is helpful for scientists from many different fields to take part in the ice dives. "It's not enough for a marine biologist to go down and count the starfish on the sea floor," he says. "We need multidisciplinary teams with a chemist and a physicist to go down and connect the dots." This enables researchers to more comprehensively describe the range of physical and biological processes that occur in these waters.

However, such fieldwork is risky. "It's dangerous. You can't make any mistakes," warns Finnish explorer Pata Degerman, who also teaches on the course. "It's like diving in a cave in a sense that the ice is a roof above your head. You can't just go up anywhere you need to find an exit hole."

Pre-dive, the trainees don special clothing to protect themselves from the extreme cold: thermal underwear beneath a dry suit, gloves, and a neoprene hood that covers their head and neck. Even so, they can't stay in the water for long. Their hands lose dexterity quickly in the freezing conditions. Instructors say most divers can't use their hands properly after about 30 minutes beneath the ice.

Your guideline is like a baby's umbilical cord. You can see you're connected and you feel safe – Perry Brandes

Divers on the training course descend into the water in pairs to a depth of 12m (39ft) while tethered to a safety line, which is standard procedure for ice diving. This safety line takes the form of a sturdy rope that physically connects divers to the surface. On the training course at Kilpisjärvi, each line is tended by one of the trainees, who is tasked with managing slack and making sure the line doesn't get entangled.

"Your guideline is like a baby's umbilical cord. You can see you're connected and you feel safe," says Brandes. 

The safety line is also the only means of communication that divers have with their colleagues at the surface. Five minutes after entering the water, a diver will pull hard on the rope to signal that they are OK and that things are progressing as planned. The tender on the surface pulls their end of the cord to acknowledge the message. "This gets repeated every five minutes during the dive," explains Degerman. "It's very simple but it works."

If there's no reply, or the tenders feel that something is wrong, emergency procedures kick in. A diver is always waiting on standby at the surface, ready to attach themselves to the line and jump in to find out what's going on. Once they reach the silent diver, they might need to give them air or even push them upwards and back out of the hole, adds Degerman. 

Among the things that can go wrong are problems with regulators, the devices divers use to breathe while underwater. The moisture in a diver's breath can actually freeze and cause the regulator to malfunction, says Degerman.

During the week-long course, there were a number of "free flows",  situations in which the regulator delivered air continuously at full flow, rather than in a controlled manner synchronized with a diver's inhalations. These free flow events are more likely when diving in cold water as regulators can freeze more easily. When a diver inhales, the regulator reduces high-pressure air from the tank to ambient pressure. In freezing water, this can cause moisture in the regulator to form ice that can jam the valve open, leading to a continuous flow of air making it hard to breathe properly. 

To remedy the problem, divers can switch over to their backup system. If that doesn't work, they have to get the attention of their diving buddy so that they can begin sharing their air supply and return to the surface.

Ice divers don't wear a full face mask because that would make it difficult to remove during a free flow incident. But this means they experience a significant cold shock upon entering the water. 

"I'd never dived in cold water before," says Kalkowski-Pope. "Going beneath the ice layer for the first time and feeling the cold water on my face was really unique."

Despite the challenges of ice diving, Norkko says he's never had an accident on any of his polar expeditions. He puts that down to preparedness, training, and assessing and managing risks carefully.

"People worry about different things but I think the biggest risk is dry suit flooding, especially in Antarctica," he says. "We have salt water there that freezes less easily: it's -2C (28F), which gives you a bad cold shock."

Seals sometimes sit over dive holes, blocking a diver's exit from the water. "You can't get past a 300kg (47st) seal," laughs Norkko. "That's why we always have two holes."

While there are clearly risks in doing this work, the chance to gather crucial data makes them worth taking, says Anni Makinen, who works as a scientific diver for an environmental consultancy in Finland: "I'd like to help to get some scientific knowledge that will influence politicians."

Ice diving research projects still need willing humans like Makinen. While robots and remotely operated vehicles (ROVs) are increasingly important for scientific fieldwork, including in the Arctic, there are things that machines will never be able to do, stresses Rodd Budd from New Zealand's National Institute of Atmospheric Research and coordinator of Antarctica New Zealand's dive operations.

"An ROV can only see what's directly in front of it, so it may go past something interesting," he explains. In some cases wild animals such as seals or white whales have been used to collect data from under the ice by attaching sensors to them, but they can't be controlled to go where researchers might want. Human divers, on the other hand, naturally take in a wider field of view and can adjust their explorations depending on what they deem most important to investigate.

Plus, humans are less intrusive, says Perry Brandes, the Floridian diver, who notes that ROVs create a lot of noise and shine powerful lights ahead of them. Human divers can be much less disruptive. "Many of the animals actually look at us divers," adds Brandes. "There's an interaction between us."

Norkko says that he and his fellow scientists are so dedicated to this work because they are aware of the urgency of climate change. At present, there is a race afoot to understand it, and to respond to it.

"Climate change is progressing at such a rate that decisions are sometimes not made with the best available scientific knowledge. This is a problem. We need to keep science at the front," says Norkko.

But there is also the alluring thrill of fieldwork like this. Going to places where few humans can, in order to document our planet a little better. That, too, says Norkko, keeps researchers like him returning to the dark world under the ice. "There is, of course, also an element of adventure that drives us."

-BBC