Initially 100 million seeds from more than 100 countries have been sent for safekeeping at the $10 million facility which holds 268,000 seed samples, each from a different farm or field.

"Biological diversity is under threat from the forces of nature ... and from the actions of man," Norwegian Prime Minister Jens Stoltenberg said at the opening ceremony.

"The seed vault is our insurance policy" against threats such as war, natural disasters or climate change, he said.

Dubbed a doomsday vault, the cavern in the Svalbard archipelago off the northern tip of Norway is a backup storage for seeds from gene banks around the globe.

The deposits range from major African and Asian staples such as rice, maize, wheat, cowpea and sorghum to European and South American varieties of eggplant, lettuce, barley and potato. Genetically modified varieties will not be included.

"We will have a major (seed) collection here, one of the biggest in the world, from the opening day," Cary Fowler, head of the Global Crop Diversity Trust which is funding the operations of the vault, told Reuters.

Stoltenberg and 2004 Nobel Peace Prize winner Wangari Maathai, a Kenyan environmentalist, put the first box of rice seed in the vault at an inauguration ceremony also attended by European Commission President Jose Manuel Barroso.

CLIMATE TREATY

"We hope and work for the best but have to plan for the worst," Barroso said.

He hoped international cooperation in setting up the vault could be matched by efforts to agree a new UN treaty to fight climate change at a meeting in Copenhagen due in late 2009.

"The conditions down here in the vault are perfect," Fowler said inside the gently sloping steel tube tunnel leading down to the three vault rooms that will be able to house 4.5 million samples, some 2 billion seeds.

"In the past, when accidents or natural disasters or war intervened and destroyed samples, then that was it -- they were as dead as a dinosaur, extinct," Fowler said.

"But we are going to put an end to extinction with this vault because we are going to have a safety backup, a Plan B."

Seeds deposited in the vault remain the property of the depositors, which include the world's major gene banks in developing countries.

During a visit to the site on Monday, whirring freezer equipment added an extra chill to the first vault room to be opened. The seeds will be kept at a storage temperature of minus 18-20 degrees Celsius (minus 4 degrees Fahrenheit).

Barley can survive 2,000 years, wheat 1,700 and sorghum almost 20,000 years under such conditions, the Trust said.

If the freezers failed, the permafrost would keep the cavern at around minus 4 Celsius, allowing time for repairs.

"I like having a Plan B to our Plan B," Fowler said.

(Editing by Robert Woodward)