The top-security repository in Svalbard archipelago near the North Pole, where construction began on Monday, will preserve some three million batches of seeds from all known varieties of the planet's crops.

This precious gene bank would make it possible to re-establish crops obliterated by major disasters, say the project's initiators.

"What we will store on Svalbard is not just one or two million seed samples and germ plasm, but the work of countless generations of farmers for thousands of years," Cary Fowler, executive secretary of the Global Crop Diversity Trust, told AFP.

Fowler said crop diversity was imperilled not just by a cataclysmic event, such as a nuclear war, but also by natural disasters, accidents, mismanagement, and short-sighted budget cuts.

Surrounded by permafrost and rock, the seed samples, such as wheat and potatoes, will be stored at a temperature of minus 18 degrees Celsius (minus 0.4 Fahrenheit), which the trust hopes will ensure their survival for hundreds, maybe even thousands, of years.

Dubbed a "Noah's Ark" by the Norwegian government, the seed bank is expected to open in September 2007.

Norwegian Prime Minister Jens Stoltenberg took part in a groundbreaking ceremony at the site on Monday.

Stoltenberg symbolically placed a tube filled with seeds at the site, closely guarded by an armed police officer ready to fend off the polar bears that roam Svalbard, 1,000 kilometers (620 miles) from the North Pole, and which are seen as a natural protection for the vault.

"The vault is of international importance. It will be the only one of its kind, all the other gene banks are of a commercial nature," he said in comments reported by Norwegian news agency NTB.

"It will be our last safety net. If the seeds stored in all the private gene banks are destroyed, which has happened about 40 times already, the contents of this vault will enable us to replace them," he said.

Norway financed the construction project, estimated at three million dollars (2.4 million euros), and is charged with running the vault, but the seeds placed inside will remain the property of the country of origin.

In an interview with AFP, Fowler underlined the importance of preserving the world's plant life.

"At the end of the 1800s, 7,000 named apple varieties were grown in the United States. Now, 6,800 of those are as extinct as the dinosaurs," he said.

"Our crops are the oldest artefacts in the world, they are older than the pyramids, and they are alive," he said.

Buried 300 meters inside the mountain, the vault will be located behind an armour-plated door. A meter of reinforced concrete will fortify the chamber walls.

Arctic permafrost will act as a natural coolant to protect the samples which will be stored in watertight foil packages should a power failure disable refrigeration systems.

The thick walls, airlocks and doors mean that even if global warming accelerates badly, it would take many decades for hotter air to reach the seeds.

"It will ultimately house replicates of every known crop variety, as well as have ample capacity to accommodate new variation as it arises naturally," according to the Global Crop Diversity Trust.

Some of the 1,400 gene banks scattered around the world are in developing countries and could come under threats such as famine, natural and man-made disasters.

While the seed banks' status varies greatly, many are in dire straits, the Trust said, threatening the survival of some of the world's unique crop varieties.

The project has drawn acclaim but also doubts in other quarters.

Jean-Marie Prosperi, an expert at France's National Institute of Agronomic Research (Inra), said the planned size of the vault, with three million seeds, was "very big." "There are between 500,000 and 700,000 in the entire United States," said Prosperi.

However, "you can't freeze seeds with the hope of germinating them a thousand years from now. The fittest can be conserved for 30, 40 or 50 years. Beyond that, the seeds die, so you have to regularly renew the collection," said Prosperi.

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Global Crop Diversity Trust