IN THE WINTER of 2008, the trajectory of Gina Moseley’s career shifted over the course of a pint in a pub. A budding paleoclimatologist doing a PhD at the University of Bristol on cave environments at the time, Moseley was meeting with members of the university caving society, when she struck up a conversation with longtime cave explorer, Charlie Self.

He began telling her about a remote cave in north Greenland, a vast, gaping black hole embedded in the crest of a steep cliff hundreds of metres aboveground and seemingly impossible to reach. In the 1960s, the United States Army took the first official photographs of the cave during Cold War reconnaissance missions, when pilots were seeking emergency ice-free landing strips. Since then, a handful of researchers — Self included — had tried but failed to reach it. Some had even gone so far as to hover a helicopter at the mouth to peek inside. None of them had made it in.

Caves contain some of the most reliable cues about past climates, captured in stalactites and stalagmites, the sculptural formations that often decorate their ceilings and floors. To the young paleoclimatologist Moseley, hearing about this unexplored chasm in the swiftly changing climate of north Greenland was tantalising. “I was immediately excited by the idea of an expedition,” Moseley says. Self loaned her the tome of information he’d gathered, which Moseley photocopied before resigning the pages to a drawer. “I didn’t know what to do with it,” she says. Over the years, her mind would occasionally turn to that cave, and she’d imagine the expedition she’d take to reach it. “But I didn’t have the confidence — or even know where to start.”

Over a decade later, with three interim trips to Greenland’s more southerly caves under her belt, and a generous batch of funding, that’s all about to change. Moseley — now a seasoned caver, polar explorer, and paleoclimatologist at Austria’s University of Innsbruck, is preparing to lead a world-first, six-person expedition up to the Arctic Circle to explore that enigmatic cave. The team’s research on its contents will provide a glimpse into the deep history of Earth’s northern extremes. These regions are now heating up twice as fast as anywhere else on the planet: knowing what they once looked like could help us predict the risks to humanity if they continue melting, shifting global temperatures and driving catastrophic sea level rise.

MOSELEY’S LOVE FOR caves began on a family camping trip to Cheddar, Somerset, when she was 12 years old. When an opportunity came up to take a guided tour of some local caves, she made her first foray into the dim, labyrinthine underbelly of the earth’s crust. Crawling about in the muck, she was instantly hooked: she started saving the money from her paper round so that on future holidays to Somerset she could disappear into those caves once more. “I just absolutely loved it,” Moseley says. “Other people might get the feeling if they go to Venice, and they wander around the alleyways, and want to know what’s around the corner – you know, that excitement. I get that when I’m underground.”

Years later she started a PhD in paleoclimatology, which combined her two interests: caving, and climate change. Caves are valuable in climate research, because many contain speleothems: the collective name for geological features that are made out of mineral deposits, forming the stalagmites, stalactites and flow stones that hang from the cave roofs, or rise up like anthills from the floor. Speleothems grow slowly over millennia through the gradual drip of water from the outside world through the cave’s roof. Mineral calcite deposits form the physical structure, building individual hair-thin layers over time, somewhat like tree rings. “Each drop of water brings with it a chemical signature that can tell us about the processes going on at the surface at the time it was deposited,” Moseley explains. Calcite, oxygen, carbon, even traces of soil, pollen, and vegetation captured within, collectively build up a picture of past environments: what the carbon dioxide levels were in the atmosphere, the temperature, rainfall levels, and even the cave’s surrounding habitat.

Meanwhile, the cave itself keeps this valuable record highly-preserved, making them portholes to the past. “[Caves are] well-connected to the surface environments, but also well-protected from the surface environment. What that means is that they’re sitting there beneath the surface, silently recording changes that are taking place over hundreds of thousands, even millions, of years,” Moseley says. This stable environment can generate long, detailed records of past climates that are largely intact. That’s compared to other climate archives like marine sediments which may be more vulnerable to disturbance by animals, or ice cores, which melt away when temperatures warm. “[Speleothems] really do tick many boxes, and provide some nice advantages as an archive type,” says Christopher Day, an isotope geochemist and paleoclimatologist at the University of Oxford, who is not involved in Moseley’s research.

Advances in the uranium thorium radiometric dating used to analyse speleothems now allow researchers to date individual layers down to an accuracy of roughly 20 years. Paired with the environmental information they contain, “we can say with a lot of certainty when certain things happened,” Moseley says. She’s careful to emphasise that when it comes to climate records, speleothems are just one small part of a bigger picture but because caves are spread widely across the planet, their speleothems can help fill the gaps where other archive measures don’t exist. Meanwhile, their detailed records help build a richer global portrait of past environments. “You can gradually work towards a much more global representation of what the environment looked like, but with information that is specific to individual regions,” says Day. That helps researchers detect broad climate trends, and also compare the differences and relationships between regions, he explains.

Because some features of past climates mirror the warming we’re experiencing now – and will face in the future – those records provide an invaluable guide to our future climate, Moseley says. “I am trying to tap into these periods in the past when it was warmer than today, in order to give us a real-world example of what the climate and environment might be like.”

So far, her hunt for speleothems has taken her around the world. Moseley has rowed across an underground cave lake, beneath chandeliers of dangling stalactites in the Grotte de Gournier in France. She’s rappelled down a 152-metre rock face to take samples from the bottom of the Devil’s Hole Cave in Nevada. She was part of a team that gathered stalagmites from the underwater Crystal Caves of the Bahamas — all featured in an IMAX documentary about her work. But, some of the most revealing clues about the future impact of climate change on our planet lie waiting in the frozen caves of north Greenland. That’s what keeps drawing Moseley back to this hostile, and almost unreachable place.

MOSELEY’S FIXATION WITH Greenland reignited when she finally went back those files from Charlie Self years after he first entrusted them to her. Nestled within the sheaf of the papers, she found a study on a single flowstone — a speleothem formed by sheets of constantly flowing water — from a cave in northeastern Greenland. “And at that point, [the idea of a trip] became more interesting to me, because I knew there was at least one sample in Greenland that might hold some physical climate information.”

This lightbulb moment occurred in a context of extremely limited scientific knowledge about Greenland’s northeast. The risk and cost of travelling through this harsh, lonely terrain had deterred most Arctic explorers; the region has no permanent human inhabitants, except for five people who manage a Danish military base there. “Even at the start of the last century, we didn’t know what the coastline of northeast Greenland looked like because it hadn’t yet been mapped,” Moseley says. When it came to caves — let alone speleothems — knowledge was scant.

But Moseley was driven by the belief that geological samples from these far reaches could provide uniquely important insights into global climate change. “The Arctic is warming up twice as fast as the global average, so it’s highly sensitive to climate change, and we need more information about what we might expect in the future,” she says. For instance, even the mere presence of speleothems in the now-icy Arctic Circle tells us it has previously been wetter and warmer than today, since there had to have been running water for the speleothems to form, Moseley explains. Many periods in Earth’s history were previously warmer, driven by factors such as its position relative to the sun. Some of these hotter periods created a very different world than the one we live in today, where for instance sea level was significantly higher and Earth would have been less habitable for humans.

Crucially, these changes unfolded on different geographic and time scales than the rapid human-induced changes we’re experiencing now. But taking clues from this region’s warmer past could help us understand the consequences of the warming that’s now accelerating in the Arctic — and how melting in polar regions could, for instance, intensify global sea level rise. Paul Smith, a geologist, director of the Museum of Natural History in Oxford, and long-time collaborator on Moseley’s work, describes it this way: “Greenland is returning to a much warmer state than any human has ever known. If we’ve got [past evidence of] deciduous trees growing at the North Pole, what’s it going to be like at the equator?”

Emboldened by the promise of that single flowstone, Moseley took her first of three expeditions to northeast Greenland in 2015. Without any evidence to present, that initial venture was difficult to fund through traditional routes — so the team drew on donations from 59 private sponsors to get them into the field. The journey, which included Moseley and four colleagues — one being her partner Robbie Shone, a National Geographic contributing cave photographer — began with a gruelling, triathlon-style trek across northeast Greenland’s frigid, rocky terrain. A twin otter plane deposited them on a remote landing strip, which was followed by a boat trip across a vast lake, then a three-day hike that ended with frantic scrambling up shifting scree slopes to reach the caves the team was there to explore.

Once they reached their destination, they were rewarded far beyond their expectations. “We learned there were far more caves than we ever could have imagined, and they were absolutely full of speleothems,” says Moseley — so much so that the team ended up having to offload food to make space for the precious cargo they’d gathered from the caves’ depths. “We were replacing freeze-dried food with lumps of rock,” Moseley says.

Two more trips followed in 2018 and 2019 to northeast Greenland, and with an accumulated five years’-worth of speleothem samples, the team have so far made several key findings. The most recent was published this year in the journal Science Advances: some of the speleothems they gathered date back as far back as 588,000 years ago, to warmer periods when ice core records are unavailable. Nestled in the laminae was evidence of warming marked by temperatures of 3.5 degrees warmer than today, which led to increased rainfall and permafrost thawing – a process associated with the release of more carbon dioxide into the atmosphere.

It’s a vision of a world we might see again, under the very different, anthropogenic regime of climate change. And embedded in that enigmatic cavern in the mountains of Greenland’s far north, there could be clues that reach even further back into history.

IF YOU PICTURE Greenland like a hand, says Paul Smith, then the cave sits on what looks like a finger of terrain called Wulff Land, which seems to point emphatically up at the North Pole. He’s one of a handful of people who’ve seen the cavern relatively up close, in his case from the window of a precarious helicopter back in 1984. At the time, Smith was in Wulff Land on his first geological field trip to Greenland, and on his flight back to base camp, he asked the pilot to swoop by the cave so he could peer within.

“There was a vicious updraft, so we were really being bounced around,” he recalls. But what struck him up close was the sheer volume of the hole; Smith reckoned it would have been large enough to accommodate the helicopter. This in itself was a tantalizing clue to past climates. “When was there enough water to generate a cave passage big enough to land a helicopter in?” Smith says.

Decades later, Smith would pass some of the photographs he took that day into Moseley’s hands — and those enigmatic pictures were part of the reason the cavern remained imprinted on her memory for so many years. By that point, Charlie Self had passed away and sadly didn’t get to see even the first 2015 expedition, Moseley says. But now, she has received an Award for Enterprise from Rolex, which will finally make possible the expedition that Moseley — and Self — had dreamed of for so long. It will cover the prohibitive costs of getting to remote Wulff Land and crossing the harsh terrain with a team of six people, which will include Moseley, her partner the photographer, a rope access expert — and a doctor. (They’ll be so far away from help that if someone gets hurt and the weather systems change, “we could be a week or two away from being picked up,” Moseley explains.)

As she speaks, Moseley is in the midst of planning the journey: the epic trek from base camp to the cave will culminate in a hike to the top of the plateau of the cliff where it’s embedded; following that will be “something like a 1600-metre abseil from the top to get in,” Moseley says. “Then there’s just this big black hole. That’s all we can see at the moment.” (She will also visit a few other caves in the area to expand the sample base of regional speleothems.)

Moseley can’t be sure of what she’ll find inside the mysterious cave. On the one hand, her quest is driven by this uncertainty: “There’s this idea that this cave was photographed in the Cold War and still not explored, and [is] one of the most northerly caves in the world,” she says. “Who wouldn’t want to know what’s in there? I’m just so excited to find out.”

On the other hand, there’s also the potential trove of unique data that speleothems from this cave could contribute to our logbook on future climate change. “The caves in Wulff Land are a good 600 or 700 kilometers away from the ones in northeast Greenland. They’re far enough away that they might respond very differently to climate change,” Moseley explains. They’re also positioned on the northern prow of Greenland that butts into the frozen Arctic Ocean, which might alter the effect on this landmass under warming temperatures. Bringing data from these caves into the fold provides “added value for our knowledge on climate change,” Moseley says.

She also nurses a slim hope that the caves might contain preserved speleothems that, in combination with a special dating method, could allow her to look significantly further back in time. “In theory, we might also be able to go as far back as three million years. I don’t focus on this, because it’s a less guaranteed output of the project — but it’s still possible,” Moseley says. Ultimately, the cave’s location will make its data unique: if records reveal that landscapes at this frozen, absolute northern extreme were once wet and warm, what can this reveal to us about the global consequences of Northern Greenland heading for rapid, anthropogenic ice loss?

For Moseley, contributing the beginnings of an answer will require months of expedition planning, and weeks of sampling in harsh extremes, followed by possibly years of analysis. Against this backdrop of uncertainty, risk, and difficulty, what compels her to keep marching into the belly of caves?

Aside from her love for these environments, she now has another reason to keep looking for clues to past climates, she says: her five-month-old child. “I’m really invested in the bigger research question. But I feel like it became even more important to me after I had my daughter,” Moseley says. “By the time she’s grown up, there won’t be any glaciers left in Europe to see, and I find that really sad.”

Reading speleothems in caves is an art in documenting what we could lose. Yet, it also shines a spotlight on what we need to protect. Before her daughter was born, Moseley had to put together a thesis on her independent research since her PhD, cataloguing the caves she had studied and which have defined the course of her research life. After she gave birth, she went back and edited the prologue to include her daughter there: “The reason for this whole body of work, I now realise, is for her and her generation,” Moseley says. “I do believe that.”