Near the Yana river basin, in a vast area of permafrost, there is a dramatic tadpole-shaped hole in the ground: the Batagaika crater.

The crater is also known as a "megaslump" and it is the largest of its kind: almost 0.6 miles (1km) long and 282ft (86m) deep. But these figures will soon change, because it is growing quickly.

Locals in the area avoid it, saying it is a "doorway to the underworld". But for scientists, the site is of great interest.

Looking at the layers exposed by the slump can give indications of how our world once looked – of past climates. At the same time, the acceleration of the growth gives an immediate insight into the impact of climate change on the increasingly fragile permafrost.

There are two types of permafrost. One is from glacier ice, left over from the last Ice Age and now buried underground. The other type, the one present around the Batagaika crater, is ice that has formed in the ground itself. Often, this ice is trapped beneath a layer of sediment and has been frozen for at least two years.

The Batagaika crater opens up a vast area of previously buried permafrost, some of which first formed many thousands of years ago.

During the last 200,000 years, Earth's climate has alternated repeatedly

The trigger that led to the crater started in the 1960s. Rapid deforestation meant that the ground was no longer shaded by trees in the warmer summer months. This incoming sunlight then slowly warmed the ground. This was made worse by the loss of cold "sweat" from trees as they transpire, which would have kept the ground cool.

"This combination of less shading and less vapid transpiration led to warming of the ground surface," says Julian Murton of the University of Sussex in the UK.

As the ground surface warmed up, it caused the layer of soil right above the permafrost to warm. This caused the permafrost itself to thaw. Once this process started and the ice was exposed to warmer temperatures, melting escalated.

For these reasons, scientists are actively monitoring the crater. One study, published in the journal Quaternary Research in February 2017, found that analysing the layers now exposed could reveal 200,000 years of climatic history.

During the last 200,000 years, Earth's climate has alternated repeatedly between relatively warm "interglacial" periods and chilly "glacial" periods in which ice sheets expanded.

The Batagaika sediment layers provide a "continuous record of geological history, which is fairly unusual," he says. "That should allow us to interpret the climate and environmental history there."

The climatic history of a huge part of Northern Siberia is little understood

However, for now the dates are not certain. "We are still working on the chronology," says Murton.

Next, he needs to gather and analyse more sediments. Ideally, these will be collected using a drill in order to get a "continuous sediment series", which will help give more accurate dates. The permafrost record could then be compared with data from other temperature records, such as ice-cores from ice sheets.

"Ultimately, we're trying to see if climate change during the last Ice Age [in Siberia] was characterised by a lot of variability: warming and cooling, warming and cooling as occurred in the North Atlantic region," says Murton.

This is important, because the climatic history of a huge part of northern Siberia is little understood. By reconstructing environmental changes that happened in the past, scientists could help forecast similar changes.

For example, 125,000 years ago, the climate was going through an interglacial period, during which it was several degrees warmer than it is now. "If we can understand what the ecosystem was like then – that might give us some inkling into how the environment may change now if the climate is warming," says Murton.

We now know that this permafrost is changing fast

If the permafrost responds to warming in a similar way as it did after the last Ice Age, we can expect to see many more slumps, big basins and lakes forming.

New land may even start to appear, as thawed ice exposes buried land some 33-66ft (10-20m) below the original surface. "You have this very ice-rich permafrost that starts to thaw from the top downwards as the ice drains and melts, and so we get a new landscape develop," says Murton.

These impacts may not be all that far off. We now know that this permafrost is changing fast.

Frank Günther of the Alfred Wegener Institute in Potsdam, Germany, and colleagues have been monitoring the site for the last decade, using satellite images to measure the rate of change.

Continuous growth means that the crater gets deeper and deeper every year

During their study, the head wall of the crater has grown by an average of 33ft (10m) per year. In warmer years, the changes have been even greater, sometimes up to 98ft (30m) per year. Günther announced these findings at the American Geophysical Union meeting in December 2016.

He also has reason to believe that the side wall of the growing crater will reach a neighbouring eroding valley in the coming summer months. This in turn will "very likely" be a new trigger for more growth.

"On average over many years, we have seen that there's not so much acceleration or deceleration of these rates, it's continuously growing," says Günther. "And continuous growth means that the crater gets deeper and deeper every year."

This has other worrying consequences.

Many of the ice deposits that are now being exposed formed during the last Ice Age. This ground ice contains a lot of organic matter, including plenty of carbon that has been locked away for thousands of years.

"Global estimations of carbon stored in permafrost is [the] same amount as what's in the atmosphere," says Günther.

There is no indication that the erosion of this crater will slow down any time soon

As more permafrost thaws, more and more carbon is exposed to microbes. The microbes consume the carbon, producing methane and carbon dioxide as waste products. These greenhouse gases are then released into the atmosphere, accelerating warming further.

"This is what we call positive feedback," says Günther. "Warming accelerates warming, and these features may develop in other places. It's not only a threat to infrastructure. Nobody can stop this development. There's no engineering solution to stop these craters developing."

There is no indication that the erosion of this crater will slow down any time soon, as it continues to grow year on year.

That makes the future of Siberia's permafrost look very wobbly indeed.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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