Deep underground in a reinforced vault beneath the Sussex countryside, sits one of the most biodiverse rooms in the world, according to scientists.

Inside are more than two-and-a-half billion plant seeds, carefully preserved to protect wild plants from extinction.

Each glass jar and packet of foil contains a collection gathered from forests, deserts and grasslands around the world.

It's aim: to be a global archive to secure the future of life on Earth.

The seed bank was opened in the year 2000 by King Charles, then the Prince of Wales.

It was designed to be fire resistant, flood resistant and secure enough to protect the collection through any disaster.

Since then, it has become the center of a global conservation partnership, with 279 organisations collaborating to collect, store and study the world’s wild plants.

The journey of these seeds often begins thousands of miles away.

Field collectors in places such as Madagascar, Thailand, Greece and Arctic Sweden gather samples from wild populations and send a small share to Kew for long term storage.

When they arrive, the first step is drying.

In a controlled environment at exactly 15 degrees Celsius (59 degrees Fahrenheit) and 15% humidity, seeds are left for up to three months.

The aim is to remove moisture slowly and evenly, because drier seeds live longer.

Once dried, the seeds move to the cleaning room.

Here, scientists separate them from leaves, husks and dust using sieves, brushes and a machine called an aspirator that uses air to lift away lighter fragments.

The cleaned seeds are then tested for life in the germination laboratory.

In petri dishes and small tubes, they are given moisture, warmth and light that replicate the natural conditions of their native habitat.

Only seeds proven to be viable are sealed in foil or glass and moved to the underground vault, where the cold and dryness extend their lifespan by centuries.

Scientists say that for every 1% reduction in moisture and every five degree drop in temperature, the lifespan of a seed doubles.

Some species cannot survive freezing, such as tropical trees with high water content.

For these, researchers are developing cryopreservation methods that use liquid nitrogen to store tissues safely at even lower temperatures.

Since it opened 25 years ago the Millennium Seed Bank has been involved in restoring habitats in the United Kingdom and abroad, using stored seeds to help reintroduce native plants and strengthen vulnerable ecosystems.

Researchers also work with international partners to share techniques in seed storage and germination, aiming to improve global efforts to conserve threatened species.

To fund the work, Kew has launched a 30 million pound ($40 million) Seeds Future Fund to support new projects and expand seed science around the world.

In addition, to mark its 25th anniversary, Kew is releasing a new podcast series featuring actor Cate Blanchett, to highlight the project’s global partnerships and the science behind seed conservation.

With nearly half of all flowering plants now thought to be at risk of extinction, scientists say protecting seeds is vital insurance for the future of global biodiversity.

From tiny orchids to giant palms, it's hoped that this frozen library of species may one day help to restore the planet’s ecosystems.

AP video and production by Mustakim Hasnath