The UK's Millennium Seed Bank, dubbed the 'most biodiverse place on Earth' by scientists, is celebrating its 25th anniversary.

The seed storage facility is the largest of its kind in the world and is in the grounds of Kew's Wakehurst wild botanic garden in Sussex, southern England. It was opened in 2000 – since when, scientists have collected around 2.5 billion seeds from 40,000 plant species.

The seeds are cleaned, dried and then put in storage in an underground 'doomsday' vault. Many are stored at a temperature of minus 20 degrees Celsius. This process dramatically expands their lifespan.

"They can actually then live for tens to hundreds of years beyond their normal lifespan, which is just amazing," says Dr Elinor Breman, Senior Research Leader in Seed Conservation. "It means we've not only prevented them going extinct, but we can use those seeds now and for hundreds of years to come."

When the seed bank opened 25 years ago, the aim was to collect 10 percent of earth's wild plant species – a target achieved in 2009. Now the focus is on the quality of the seeds as much as the quantity.

"Since then we've changed the angle so we're now looking to store more endemically important plants and economically important plants," says Millennium Seed Bank curator David Hickmott. 

"So rather than going for large number targets, we're actually trying to concentrate more now on useful species that are going make more of a difference."

The Millennium Seed Bank has an extensive partnership with more than 100 countries. That list includes China, which has been involved from the beginning.

Earlier this year, the China Academy of Sciences and The Royal Botanic Gardens, Kew signed a 10-year extension to their 20-year partnership. China is home to Asia's largest seed bank – the Germplasm Bank of Wild Species, or GBOWS – at the Kunming Institute of Botany.

"I was only there just last year to kind of see what the next phase in our partnership is, because they have amazing facilities and are doing some wonderful work," says Breman.

"We've got over 100,000 collections, testing them every 10 years – that's an exponential number of tests we need to do, we don't have the resource to do that. So with GBOWS, we're looking at other ways that we might be able to check the viability of those collections."

Plants produce the oxygen we breathe, are a key part of the food chain and important for medicines and building materials. But 45 percent of flowering plant species are at risk of extinction. Climate change, pollution and overfarming are contributing to a loss of biodiversity, so there's still a lot of work to do.

The Millennium Seed Bank has launched a $40 million future fund to support its next 25 years of work and people are able to adopt a seed to help with the fundraising effort.

So what is the scientists' biggest hope for the next quarter century?

"Maybe by the end of those twenty five years we'll have put ourselves out of business," says Dr Breman. "It sounds crazy, but I would like a world where the plants are safe in their natural habitat, not in a seed vault."

But for now, it's a race against time to protect the world's precious plant life.