Built with Fort Knox-type security, the depository will preserve some three million seeds representing all known varieties of the world's crops at sub-zero temperatures.

"This facility will provide a practical means to reestablish crops obliterated by major disasters," Cary Fowler, executive secretary of the Global Crop Diversity Trust, said in a statement.

He 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 will ensure their survival for hundreds, maybe even thousands, of years.

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

Norway's Prime Minister Jens Stoltenberg took part in a groundbreaking ceremony at the construction site on Monday.

Guarded by an armed police officer ready to fend off the polar bears that roam the Svalbard archipelago, 1,000 kilometers (620 miles) from the North Pole, Stoltenberg symbolically placed a tube filled with seeds at the site.

"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," Stoltenberg said in comments reported by Norwegian news agency NTB.

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.

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

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.

"Yet agriculture worldwide relies on these collections of crop species and their wild relatives. They are vital to the development of new varieties, without which agriculture would grind to a halt.

"We need viable collections of crops like wheat, potato, and apple in areas where they originated and are still grown today," Fowler said.

"The Arctic vault and other collections around the world will make sure that the resources will be there when and where they are needed."

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