By Mark Kinver
Science and nature reporter, BBC News
The final design for a "doomsday" vault that will house seeds from all known varieties of food crops has been unveiled by the Norwegian government.
The Svalbard International Seed Vault will be built into a mountainside on a remote island near the North Pole.
The vault aims to safeguard the world's agriculture from future catastrophes, such as nuclear war, asteroid strikes and climate change.
Construction begins in March, and the seed bank is scheduled to open in 2008.
The Norwegian government is paying the $5m (£2.5m) construction costs of the vault, which will have enough space to house three million seed samples.
The collection and maintenance of the collection is being organised by the Global Crop Diversity Trust, which has responsibility of ensuring the "conservation of crop diversity in perpetuity".
"We want a safety net because we do not want to take too many chances with crop biodiversity," said Cary Fowler, the Trust's executive director.
"Can you imagine an effective, efficient, sustainable response to climate change, water shortages, food security issues without what is going to go in the vault - it is the raw material of agriculture."
The seed vault will be built 120m (364ft) inside a mountain on Spitsbergen, one of four islands that make up Svalbard.
Dr Fowler said Svalbard, 1,000km (621 miles) north of mainland Norway, was chosen as the location for the vault because it was very remote and it also offered the level of stability required for the long-term project.
"We looked very far into the future. We looked at radiation levels inside the mountain, and we looked at the area's geological structure," he told BBC News.
"We also modelled climate change in a drastic form 200 years into future, which included the melting of ice sheets at the North and South Poles, and Greenland, to make sure that this site was above the resulting water level."
By building the vault deep inside the mountain, the surrounding permafrost would continue to provide natural refrigeration if the mechanical system failed, explained Dr Fowler.
'Living Fort Knox'
The Arctic vault will act as a back-up store for a global network of seed banks financially supported by the trust.
Dr Fowler said that a proportion of the seeds housed at these banks would be deposited at Svalbard, which will act as a "living Fort Knox".
Although the vault was designed to protect the specimens from catastrophic events, he added that it could also be used to replenish national seed banks.
"One example happened in September when a typhoon ripped through the Philippines and destroyed its seed bank," Dr Fowler recalled.
"The storm brought two feet of water and mud into the bank, and that is the last thing you want in a seed bank."
Once inside the vault, the samples will be stored at -18C (0F). The length of time that seeds kept in a frozen state maintain their ability to germinate depends on the species.
Some crops, such as peas, may only survive for 20-30 years. Others, such as sunflowers and grain crops, are understood to last for many decades or even hundreds of years.
The Arctic conditions will help keep the seeds in a frozen state
Once the collection has been established at Svalbard, Dr Fowler said the facility would operate with very little human intervention.
"Somebody will go up there once every year to physically check inside to see that everything is OK, but there will be no full-time staff," he explained.
"If you design a facility to be used in worst-case scenarios, then you cannot actually have too much dependency on human beings."