"You are standing in the world's most biodiverse location," proclaimed a yellow poster at the door of the largest global collection of wild plant seeds, sunk in an English hillside.

The underground concrete vault in West Sussex is part of the Millennium Seed Bank, where rows of steel shelves lined with glass jars store more than 40,000 plant species at risk of extinction as climate change pressures staple food supplies.

"We are fighting a genetic bottleneck," said Christopher Cockel, a project coordinator at the seed bank managed by the Royal Botanic Gardens, Kew.

"Eighty percent of our calorie intake comes from 12 domesticated plant species," he emphasised.

Historically, farmers cultivated at least 7,000 different plants to eat. But since the 1960s, they have focused on higher-yielding crops to produce more food using fertilisers, chemicals and new irrigation methods, said researchers at the seed bank.

That has come at a cost to biodiversity while increasing contamination of drinking water and soil degradation.

The Earth's loss of genetic diversity means crops are less resilient to disease, invasive species, pests, habitat loss and climate change, said the researchers.

One million animal and plant species are at risk of extinction due to humankind's relentless pursuit of economic growth, said a recent report by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES).

Based on 15,000 scientific papers, it identified threats from the disappearance of insects vital for pollinating food crops, to the destruction of coral reefs that support fish populations and the loss of medicinal plants.

'NOT NOAH'S ARK'

The seed collection was started in 1978 in the fridge of an old mansion. Today the Millennium Seed Bank contains 2.3 billion seeds gathered from 35 biodiversity hotspots around the world.

"This is not rocket science," said John Dickie, head of seed collections at the bank.

"We are simply slowing down the chemistry of the seed through drying and cooling. It is what people have done for thousands of years."

Seeds are sent from partner researchers in 190 countries and territories, often via simple courier package delivery.

The seeds are then cleaned, X-rayed to check for insect larvae, dried and put into airtight glass containers before being placed in a vault chilled at minus 20 degrees Celsius.

Every five to ten years, they go through a germination test to see whether they have survived the freezing.

"This is not a Noah's Ark scenario," said Dickie. "We want to test and use these seeds now for research."

Above and below ground, scientists in lab coats can be seen at work behind glass doors, bent over microscopes as they prepare each seed for storage or research.

Many other team members are scattered around the world, working with local communities to understand which crops are most intertwined with their daily diets and livelihoods.

"We care about what is relevant to communities - such as the Morama bean in Botswana which can also be turned into milk and butter," said Tiziana Ulian, one of the seed bank's senior researchers exploring nutritional plants used in everyday lives.

The vault ships out wild seeds to gene banks worldwide where researchers grow and compare them to domesticated seeds.

"This comparison of wild and domesticated seeds under exposure to different stressors - such as heat, water or pests - allows us to look for traits in wild crops that may be more resistant to change," said Cockel.

From insects to seagrass, crustaceans and fungi, nearly a quarter of about 4,000 wild food species are in decline, with Latin America, Asia and Africa the hardest-hit regions, according to a February report from the U.N. Food and Agriculture Organization (FAO).

And climate change will become a steadily bigger threat to biodiversity by 2050, scientists warned in a 2018 U.N.-backed study, adding to damage from pollution and forest clearance to make way for farming.

Researching resilient crops that could help feed the world's growing population as the planet warms is a key motivation for the seed bank's work, said Cockel.

One in nine people do not have enough food today, while the world population is expected to reach 9.8 billion by 2050.

'PLANT BLINDNESS'

A large part of the seed bank's mission is to ignite wider public interest.

Researchers first used the term "plant blindness" two decades ago to describe modern society's disconnect from the natural environment, including difficulty in naming plants or identifying crops used for food.

Education, through visiting botanical gardens or seed banks for example, is an important tool to help reverse that, said a paper published in the journal Plants, People, Planet in May.

"We do not want a fenced-off forest," said Dickie. "In fact, what we do here is more about people than science."

On a May afternoon, visitors browsed an exhibition where they learned more about the plant species stored and researched in the vault beneath their feet. They photographed large proteas, cacti and succulents growing in the glass greenhouses.

"Unfortunately we are losing plant species faster than we can study and account for them," said Tara Moreau, a co-author of the plant blindness study and associate director of the University of British Columbia Botanical Garden.

"Public education is key," she added.

Thick concrete and carefully designed architecture protect the seed bank from radiation, bombs and floods. Alarms guard it from thieves. The only threat Dickie's team can do little about is the uncertainty of the planet's agricultural future.

"The design life of the vault should last up to 500 years," said Dickie at the end of the tour. "Who knows what will happen after that?"