Engineers last week finished work on one of the world's most ambitious conservation projects: a doomsday vault carved into a frozen mountainside in the archipelago of Svalbard, a few hundred miles from the North Pole.

Over the next few weeks, the huge cavern - backed by the Norwegian government and the Gates Foundation - will be filled with more than a million types of seed and will be officially opened in February next year.

'This will be the last refuge for the world's crops,' said Cary Fowler, of the Rome-based Global Crop Diversity Trust, which is building the vault. 'There are seed banks in various countries round the globe, but several have been destroyed or badly damaged in recent years. We need a place that is politically and environmentally safe if we are going to feed the planet as it gets hotter.'

New varieties of the world's main plant foodstuffs - maize, barley, rice, wheat and other crops - are constantly being created by plant breeders as pests develop new ways to break down a strain's natural resistance and as local climates change. 'The bread you eat today is made from very different wheat strains from the bread you had 10 years ago,' said Fowler.

The old strains - which can date back hundreds of years - are a crucial resource, nevertheless. Their seeds may prove invaluable as environments alter. Varieties discarded for more bountiful but less hardy types could regain their usefulness. By 2030, for instance, current strains of maize will no longer be able to grow in South Africa because of rising temperatures. Possessing old types of maize seed as a basic resource could be invaluable, say scientists.

National seed banks were meant to be the key defence but some have suffered serious damage, like those in Iraq and Afghanistan, which were destroyed by looters. 'The raiders were only after the glass jars in which the seeds were stored,' said Fowler. 'They destroyed an incredibly valuable resource. These countries were the birthplaces of agriculture.' Typhoons caused major damage to a seed bank in the Philippines.

The only answer, Fowler's trust decided, was to build an international doomsday vault, so the trust looked around for a site that was remote but accessible, and which was so cold that there would be no problem if the refrigeration equipment failed.

Svalbard, a group of islands far to the north of Norway but linked by a daily flight from Oslo, proved ideal. For the past year engineers using highway tunnelling equipment have drilled deep into a mountain near Longyearbyen, the main settlement of Svalbard, and have created three main vaults.

Temperatures inside fluctuate around minus 4 degrees Celsius: not quite cold enough, say seed experts. As a result, engineers are scheduled over the next few weeks to use refrigerating equipment to cool the vault to around minus 18 degrees. Then it will be ready for its seeds, say scientists.

About 500 seeds from about 1.5 million types of crop - donated by individual countries - will be placed in envelopes and about 400-500 of these envelopes are stored in a single box. Boxes will then be stacked like library books along shelves inside the vaults.

'National seed banks will still be the first line of defence for countries who need to develop fresh varieties of crops as environments change,' said Fowler. 'But if something goes wrong there, they will be able to come here, take a hundred seeds or so, and use these to grow new strains. This will be the last place we can turn to.