A cavernous bunker on a remote island above the Arctic Circle, where polar bears roam, holds the key to 12,000 years of agriculture but also to food supplies for future generations with countries urged to deposit seed samples there.

Welcome to the Svalbard Global Seed Vault, which turned 10 on Monday. It holds nearly one million seed samples from the world's gene banks - an agricultural back-up in the event of disasters ranging from nuclear war to climate change.

"It's fair to say that agriculture has never, ever faced bigger challenges than today," Marie Haga, executive director of The Crop Trust, told the Thomson Reuters Foundation.

The Crop Trust, an international group working to protect crop diversity, runs the vault in collaboration with the Norwegian government and the Nordic Genetic Resource Center

(NordGen).

Among the challenges facing agriculture, experts have said, are rising hunger, population growth and greater climate pressures.

That means the world needs to produce more food that is more nutritious, and to do so "on less land, with less water, less pesticides, less fertilizer to keep within what the planet can stand", Haga said.

The answer could lie in a modest room in the vault, measuring 12 meters (40 ft) by 27 metres, where nations have deposited seed samples of food crops for safekeeping, she said.

Shelves of boxes, stacked in neat rows at minus 18 degrees Celsius (0F), hold seeds from the United States and Russia, Australia and North Korea, and Nigeria and Colombia to name just a few.

In the decade since the vault was founded, 73 institutions have deposited crop-seeds at this so-called Bank of Last Resort.

The Crop Trust is urging other gene banks around the world to follow suit. China is the notable omission, it said, although discussions are ongoing.

Haga's concerns are echoed by Carly Fowler, a renowned American agriculturalist who helped to found the seed vault.

"Agriculture faces a historically unprecedented combination of challenges. At the top of the list is climate change," he told the Thomson Reuters Foundation.

"We're looking at climates in the near future that haven't existed in the entire history of agriculture ... We have to be proactive to make sure that agriculture does get ready for climate change."

'The Bank of Last Resort'

To do that the world needs a diverse sets of crops in its arsenal, but that is exactly what it has been losing, experts said.

"Our food system is extremely vulnerable. We are basing ourselves now on 12 plants and five animal species for 75 percent of the food we eat," said Haga, a former Norwegian politician.

Historically, farmers cultivated at least 7,000 different plants to eat. Today, 60 percent of global calorie intake comes from wheat, rice and maize, said Haga.

This loss is partly due to a focus on "productivity, appearance and taste" at the expense of other aspects such as nutrition, said Kent Nnadozie at the United Nations' Food and Agriculture Organization (FAO).

Nnadozie, who is secretary of the FAO's International Treaty on Plant Genetic Resources for Food and Agriculture, said one consequence is that a major disease or virus "could wipe out the entire crop."

To combat that risk, the Treaty - which was brokered by the United Nations - facilitates seed exchanges between global gene banks to research and develop new crop varieties. Currently, 144 countries have ratified the Treaty.

Worldwide, the FAO said, more than 1,700 gene banks of varying sizes hold collections of food crops.

But many are exposed to disasters and conflict; some have to deal with more mundane problems such as a lack of funding, poor management, malfunctioning equipment or erratic power supplies.

The loss of a crop variety is irreversible.

Ahmed Amri, from Morocco's International Center for Agricultural Research in the Dry Areas (ICARDA), knows those threats well.

In October 2015, ICARDA became the first to withdraw seeds from Svalbard after Syria's civil war had damaged a seed bank near the city of Aleppo.

The gene bank was relocated to Morocco and Lebanon, the seeds have been grown, and re-depositing began last year. On Monday, ICARDA deposited more than 8,600 seed samples. It was one of 23 institutions to hand over 77,000 samples on the day.

"These samples include wheat, barley, durum wheat and bread wheat, lentils, chickpeas, fava beans and wild relatives of these species ... So this is a big achievement," Amri told the Thomson Reuters Foundation.

The FAO's Nnadozie said the Syria example showed how well the vault could work.

"It's almost like you put your money in a long-term savings account. Once you are in a desperate situation and you need to, (you) take some money, and then you can put it back again."

"This is the final backup, should anything go wrong - natural disasters, crisis, war, nuclear, whatever - you can always go back there."

Climate pressures

The Svalbard archipelago, the furthest north reachable on a scheduled flight, was chosen for the vault's location because it is remote, there are no volcanoes or earthquakes, and the permafrost keeps the seeds in deep-freeze.

Yet the vault, built 120 meters (400 feet) into the rock, is facing its own climate pressures.

An unexpected thaw of permafrost meant water flowed into the entrance of the vault's tunnel in late 2016. The seeds were not in danger, but Norway said on Friday it would spend 100 million krone ($13 million) to upgrade the vault.

"When I came up here the first time in 1985 ... there was always ice on the fjord. Now you never see complete ice on the fjord," Haga said.

Scientists have warned that the Arctic Ocean could be ice-free much sooner than previous predictions, which forecast sea ice would first disappear completely during summer months between 2040 and 2050.

Fowler said he was confident the seeds were safe, but welcomed Norway's decision to strengthen the vault.

"We'll be tight and dry and we'll deal with whatever climate change gives us," he said.