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Instead, the challenge is with supply chains and labour markets. Workers cannot get to the farms to harvest the crops, restrictions on movements are stalling the import of goods and wholesale markets have seen their trade plummet, which together means it takes longer for produce to reach customers. As a result, the massive wheel of big farming has slowly ground to a halt 鈥� if only temporarily.

But could there be a more efficient way of doing things?

To make those supply chains shorter, for example, food needs to be grown and sold closer to customers. Much of the US, northern Europe and Canada, for example, import the majority of their fresh fruit and vegetables over the winter and spring, because customers still expect to be able to buy those products all year round.

While farming near customers sounds sensible, to farm at scale efficiently you need space. Industrial agriculture, enabled by mechanisation, has led to plummeting consumer costs, allowing more food to be available to more people for less. But big farms need big machines, with long straight lines for them to run down, which is not conducive to farming near people.

If we want to continue to feed people using farms in the places where most people live 鈥� in cities 鈥� we need to find the space for them.

One solution is to grow fruits and vegetables in space-efficient, climate-controlled indoor farms. High-tech, factory-like indoor farms have started popping up all over the world, from Japan to Scotland.

But in already over-developed cities, where can you put a massive food factory? . But there is a lot of room in cities, if you know where to look.

Underneath Paris, for example, there are 600 hectares of untapped space in the form of car parks.

鈥淚t was mandatory to have two car spaces per flat in the 1970s,鈥� says Jean-Noel Gertz, chief executive of Cycloponics. Today, with fewer people owning cars, much of this space is unused. (To put the 600 hectares of free underground space into perspective, however, the .)

鈥淧eople cannot imagine another way to use them,鈥� says Gertz. 鈥淚t is blocked in their mind.鈥�

But Gertz can. He wants to 鈥� growing organic mushrooms and endives. These two crops are perfectly suited to being grown underground. Though he acknowledges that 600 hectares is far too much space to grow mushrooms and endives exclusively.

Instead, Cycloponics is as much about underground farming as it is about providing space for other food start-ups. 鈥淲e host one NGO who gives 4,000 meals a day to hospital workers. We have another company that delivers 500-1,000 packs of vegetables to people in Paris every day,鈥� Gertz says. 鈥淲e have logistics and cold rooms, so we have everything start-ups need to feed people.鈥�

Everything, that is, to prepare food, store it and deliver it. The next step is to build small kitchens so the food can be cooked on site, too. Eventually, some foods might be grown, prepared, cooked, stored and delivered all from underground spaces in the centre of Paris.

Gertz says the most difficult challenge was first getting permission from the fire department to set up a business underground. But slowly, opinions towards urban farming are starting to change.

At the higher-tech end of the scale, some urban farmers have found innovative ways to grow a far greater variety of crops. Square Roots build farms inside refurbished shipping containers 鈥� each with its own adjustable climate.

鈥淧eople want food all over the world but the impact of transporting it is huge,鈥� says Tobias Peggs, chief executive of Square Roots. 鈥淎nd people forget the impact on the consumer because the nutrients break down in shipping. We thought, instead of shipping food, why don鈥檛 we ship climate data?鈥�

Rather than growing basil in Genova in the right climate, for example, Square Roots studies the local humidity, light, heat, soil nutrients, and length of the growing season there, then tries to recreate that in an artificial environment. That way, the same basil can be grown in the middle of a city every day of the year.

, in three years Square Roots have gone from growing herbs and leafy greens to aubergines, turnips, strawberries and tomatoes. They even had a chilli eating competition with some container-grown habaneros this spring.

The question of what you can grow, and where, is a matter of economics. Plants take in carbon dioxide, and use light energy to convert it into biomass. The heavier the veg, the more biomass 鈥� so you need more energy, which ups the expense. But leafy greens, basil, mint and chives are low in biomass. Peggs says he can get a product to market that is competitive with organic prices and in some cases with conventional farming.

鈥淚 imagine going into a supermarket and lining up fruit and veg in density 鈥� that is our product roadmap,鈥� he says. 鈥淪trawberries and tomatoes are on the cusp [of being commercially viable], we keep going from there. A field farmer cannot make the sun more efficient and halve their costs. But indoors, I can make it more efficient through technology.鈥�

Like in Paris, the US has an abundance of parking spaces 鈥� only they are largely above ground. . Add to that the threat of a 鈥�retail apocalypse鈥� as malls shut down, and the picture of millions of square feet of space begins to build. Peggs says that local building authorities already have codes for working with shipping containers, which means repurposing that space for container farms is easier than building new facilities.

鈥淵ou get a lot for free with a shipping container, they were built to transport food and maintain a climate,鈥� says Peggs. 鈥淵ou can put it on a truck, drop it in a parking lot, plug in water and internet, and you have a farmers鈥� market. There are many ways to be creative with existing infrastructure.鈥�

The internet connection is needed because Square Root鈥檚 containers are AI-powered. Each container can speak to the others to keep an updating feedback loop should conditions inside them change.

In New York, Square Roots sell to grocery retailers 鈥� all of which are within five miles of the farm. There are no trucks driving around. Instead, they have a fleet of battery-powered tricycles with a cold storage unit on the front. This means the food usually only has to travel for two hours, not two weeks.

Another underground farmer, Steve Dring, co-founder of Growing Underground, likewise saw an opportunity in one untapped urban space: . A few years ago, Dring and his co-investor were weighing up the relative values of vertical farming. Purpose-built facilities are all very well, but they can鈥檛 be built easily in the centre of a city. Meanwhile, Transport for London possessed the keys to 70,000 square feet (6,500sq m) of empty tunnels sprawling under the capital. With a little bit of convincing, they handed the keys over to Dring.

At first, their set-up looked like something from the TV series 鈥淏reaking Bad鈥�, he says. There were crops bathed in pink light (both blue and red wavelengths are used, optimal for growing, but the lights look pink) and rows of plants lining a makeshift polytunnel on a mezzanine floor, with water and filtration tanks on the floor below. Dring鈥檚 farm uses a type of hydroponics called ebb and flow, where water is pushed out to the crop and allowed to flow back through a filter.

Like Paris鈥檚 underground car parks, this wartime bunker is ideal for growing in a controlled way. 鈥淚f it is -5C [23F] or 30C [86F] above ground, it is constantly 14C [57F] down here in the tunnel, 110-120ft [34-37m] below London,鈥� he says.

Dring says that what he and farmers like him are doing has been misnamed. While the technology they are using is the same as vertical farming, Dring prefers the specific title of 鈥渃ontrolled environment agriculture鈥�. 鈥淲e control the environment to a forensic level,鈥� says Dring. 鈥淚f I want to control humidity and heat, it is easier to start from a base where it is 14C all year round.鈥�

Dring says that with a long tunnel, one of the major concerns is good airflow, otherwise pathogens build up. But the air raid shelter was designed to accommodate 8,000 people, so the space already came with 鈥渒ickass ventilation鈥�.

Now, Dring says he has spoken to every individual with an underground car park in London because he and his colleagues are trying to work out how to monetise that space in the future as car use declines.

People are starting to see the opportunities in growing in unusual urban spaces. Gertz says that the extra pressure on supply chains now has also moved the debate forward, like 鈥渏umping 10 years in the future鈥�.

鈥淭hey are giving much more respect to people working locally with their hands [as a result of lockdown],鈥� says Gertz. 鈥淭hey found out it is quite useful to have people working locally. They found out how good stuff grown locally is.鈥�

Peggs agrees, saying that in times of financial hardship, people start to consider what they eat to a greater extent. 鈥淲hen you are eating at home, you have to be looking for value; this happened in 2008,鈥� he says, referring to the previous global financial crash. 鈥淣o one cared where their food came from, they wanted cheap calories as convenient as possible.鈥�

But because this recession has been created by a health pandemic, people are questioning the choices that they are making around their food. Perhaps we will all soon be giving a bit more thought to how far our food has come from and what is in it.

This article is part of , a series investigating how agriculture is responding to environmental challenges. traces emerging answers to these problems 鈥� both high-tech and low-tech, local and global 鈥� from farmers, growers and researchers across six continents.