It is a world cut off from our own by thick blankets of floating ice, but some scientists are taking the plunge to learn how these frozen depths are changing.

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 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.

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........

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