London, England (CNN) — The subterranean world beneath London has many secrets -- hidden railways, ghostly abandoned stations, government war rooms. And now there's a farm.

There are no cows or sheep at the Growing Underground farm deep below the streets of the British capital's Clapham neighborhood. Built in an abandoned World War II air raid shelter, the unusual facility instead produces "micro greens"-- herbs and salad veggies destined for the city's restaurants.

"Micro greens" are grown hydroponically on beds of recycled carpet.

"The idea came about from my interest in sustainability, the future of cities," the farm's co-founder Richard Ballard tells CNN. With property values climbing ever higher in London, conventional farming has long since been priced out of the city's center.

But Ballard, alongside business partner Steven Dring, hit upon the idea of going below the streets of London after discovering the Clapham bunker while working as filming location scout. Located 33 meters below the surface, the deep level shelter was built between 1940 and 1942 to protect up to 8,000 people from bombing raids during World War II.

Hydroponic technology

The tunnels largely fell into disuse after the war, so when Ballard and Dring approached their owners, London's TFL transport body, they were soon invited to set up their operation. Further support came from a crowdsourced cash injection of more than $1 million and interest from Michelin-starred chef Michel Roux Jr., who signed up for deliveries to his Le Gavroche restaurant.

The plants are grown on beds of recycled carpet using hydroponics -- soil-free cultivation techniques -- and newly developed LEDs that bathe them in the light and warmth they need to flourish.

Under the pinkish glow of the artificial lights, shelves of salad leaves and herbs are now growing so successfully that supply is outstripping the demands of the local restaurant industry.

Built to shelter 8,000 people, the tunnel now houses shelves of growing herbs.

That'll mean that the subterranean harvest could soon hit superstores, allowing ordinary Londoners to buy food grown beneath their feet.

"Before the tunnels were used, they had bunk beds here, which were three or four high, they were used in a time of destruction when above ground London was being destroyed by bombs," says Ballard. "And now we're using a similar system of three or four beds to feed London."

The Growing Underground facility is also helping inspire other unusual ventures as farmers look for innovative solutions to produce food in places where land is in short supply.