There are a lot of exciting and valuable things hidden away beneath the Earth's surface: dinosaur fossils; crude oil; buried treasure. But what if I told you that deep in the bowels of London – underneath the Northern Line, in old air-raid shelter built during World War II – you'd find the possible future of urban farming?

The first thing I say when I see it for myself, after some introductions, is probably "How big?!" I'm at Clapham's Growing Underground for a quick nosy, having heard about the operation but deliberately not having done too much homework. As the rickety lift inside its tungsten-drenched headquarters slowly makes its way down to the dark recesses of subterranean London, co-founder Steven Dring tells me that yes, what I'm about to see is a "65,000sq ft hydroponic urban farm", which grows micro greens, salad leaves and herbs under LED lights. I don't know what I was expecting – a room the size of a south London weed farm, maybe (which Dring later tells me his friends and legal team have jokingly advised him to start). Whatever I'd predicted, it wasn't something the area of a football field.

That's slightly misleading – in fact, the two tunnels are long and thin, snaking from near Clapham North all the way down to Stockwell. The air-raid shelters the farm is housed in were supposed to make up a new branch of the Northern Line after the war, but, for whatever reason, they were left empty. I ask Dring to start from the top, as it were: "The whole thing came about from some studies my business partner Richard Ballard had been doing on vertical farming," he says – meaning economising on space by stacking plants on top of one another.

"It led to us deciding, instead of sitting and talking about the future of cities, to actually do something about it and get involved, and try and be part of some kind of change – whether it was democratisation of energy, or water scarcity. We were chatting about it, arguing about it, and over time that turned into this business."

Dring and Ballard knew each other from way back, having grown up in the outskirts of Bristol and Bath. It's fitting, really – near to an urban area, with the imprint of agriculture not far away. "Rich came up to London, and he started becoming more engaged with activism," Dring explains, adding that at that time he was entrenched in a "boring corporate career". Through reading up on the democratisation of energy, as well as location scouting for a film project about hidden London, Ballard realised that underground farming might actually be the way forward. "It was working out quite expensive to convert skyscrapers, because you've still got the rent," he says, "but underground was cheap. This farm costs just over £1 per sq ft."

When you're building a business, sustainability is a given

The cost may be a clear benefit when it comes to using an underground space to grow plants, but there's also a pretty clear problem: the sun – or, rather, the lack thereof. Thankfully, Ballard's research also led him to discover that technology was already making headway in precisely this field: "I also did a lot of research about LEDs – how people weren't just using them for supplementary growing; they were using them purely for growing in a controlled environment. I realised that was possible, and spoke to Steve and he said we need to start with something that was going to be profitable – micro greens was that product."

Which brings us to the specifics of the operation. Dring and Ballard insist that it's not the aim of them, their head grower and their small team of farmers to eventually phase out outdoor farming. Nor do they have the capability to do so. Dring tells me that they're growing salad leaves – and delicious ones, at that – but that when it comes to other vegetables, "do that in a field, bring that in in an efficient manner."

Although, due to the intricacies of hydroponics, Growing Underground can't be officially classed as organic in Europe – something Dring says the EU is currently looking at, though that isn't the case in the US – the potential upshots of this type of growing are clear. "These products," he says, referring to the type of salad leaves that favour warmer climates, "we don't need to be flying them in from around the world. We can grow them hyper-locally. If they're perishable, let's get them to the customer as good as we can, with the highest amount of nutritional value," he says, adding that traditional farmland is still a better environment for other vegetables.

What Growing Underground can do now, though, is to set a huge precedent for sustainable urban farming. The ecological side of the business is one that's been there since its outset, and the main reason for setting it up. It hasn't been diluted in the months from planning to execution: "We have a carbon consultant," Dring explains. "He goes through every invoice and says, 'Right, that pack of screws has a carbon impact of x''.' We carbon-account every single thing that goes into our farm, so when we publish our accounts in June this year, we'll have a set of accounts that tell us exactly how much carbon we used last year.

"Different people will argue about the virtues of carbon neutrality, but that's our way in terms of our sustainable goals. When you're building a new business, sustainability is just a given – you embed that as part of your business from the start. You get your energy from a good energy supplier, you lessen your carbon impact as much as possible, you do all your recycling. Then, at the right time, you hopefully spit out a profit for your investors and grow the business, so there's the financial dividend.

"We're around a third of the way towards getting to the point where everyone's happy. That means making a profit and making a return for our investors – that's the next bit."

Growing Underground, the farm, and Growing Underground, the brand, are to an extent separate but equal. The farm sets the example; the salad makes the money. And the product is doing well – they've just entered the first proper phase of industrial-level production, a daily delivery goes to New Covent Garden Market, and the brand is also distributed through Ted's Veg at Borough Market and Farmdrop, the online produce market, as well as a few others.

Growing Underground's products – in pictures:

Dring and his team have benefited from a helping hand in the form of a certain two-Michelin-starred chef. Michel Roux Jr, like me, became interested in the farm simply through the curiosity of sharing a neighbourhood with an improbably large, but totally hidden, urban farm.

"He was interested just in seeing the tunnels," Dring says. "He came down, saw the produce, tasted it and said "Let's do it". The whole Roux clan ended up coming to visit, with all of them effusive in their praise for the project. "We just wanted to be advised," Dring says, "'Yes, that tastes good,' 'Don't do that,' 'Grow more of that,' 'That's a little bit woody,' 'The flavour was better last week,' or whatever it may be. Having somebody with a great palate advising you on your product – that's what you want."

But Roux quickly took on more of a role than simply casual advisor. He's by now a non-executive director, with what he calls a "tiny, tiny share" in the business. Back at Le Gavroche, I ask Roux where he thinks the farm fits in the grand scheme of things. He's characteristically ebullient: "If they crack this, there are tunnels all over London, for a start. But all over Britain, all over the world, there are places that could be used. There are many, many issues concerning this but, simply put, we're not going to have enough farmland to feed the world. This can be an economical way of producing food. It's energy-efficient because these new LED lights are state-of-the-art. It's non-polluting. It just ticks so many boxes."

It's energy-efficient; the LED lights are state-of-the-art. It’s non-polluting. It ticks so many boxes

"It starts with that really big-picture stuff about growing populations," Dring says, back in the small office that sits atop the possible site of a green empire. "But simply, what we're doing is creating a fresh produce brand.

"You start with all of that big stuff, and then you're saying 'Can we feed people in a different way?', 'Can we grow food that doesn't have to travel?' Effectively we take out seasonality, which means we don't have to fly stuff in from around the world – we can do it locally. You take out food miles because you can get it there quicker, and if it lasts in our customers' fridges two days longer, it starts to affect food waste, too.

"It's all that big stuff," he says, "but really it just comes down to growing some salad. And that's exactly what we do."