The climate crisis is making cities uninhabitable. It's time to go underground.

Underground cities were long the stuff of science fiction, but recently cities and urban planners have begun to take the idea seriously. The biggest challenge is showing people that they can feel comfortable underground.________________________________________Original text by: STEPHEN ARMSTRONG________________________________________Originally published on Tuesday 12 November 2019Around 1800 BC, the people of Cappadocia, in present-day Turkey, agreed that their surroundings were too inhospitable.

Besides extreme weather, they faced the constant threat of war. So they decided to dig a city underground. Derinkuyu, the oldest underground city, still exists. They built a city for 20,000 people, with schools, homes, shops and workplaces protected by large stone doors that could seal off each level separately.

In 2010 Helsinki tried the same. The city council approved the Underground Master Plan, completed in 2019, which covers the city's entire 214 square kilometres. It combines energy storage, shelter from the long, harsh winter and ready-made bunkers in case of Russian aggression.

But it is not only security and shelter from the weather that have led people underground. Living below the surface offers a growing population an alternative to large high-rise blocks. Asmo Jaaksi, a partner at the Helsinki architecture practice JKMM and chief architect of the city's underground Amos Rex museum, says that underground spaces retain heat and can be a safe refuge if the climate crisis escalates.

Helsinki has long experimented with living underground

Temppeliaukio Church, designed by the architects Timo and Tuomo Suomalainen, was "sunk" into the Töölö district in 1969. In 1993 came the Itäkeskus swimming pool, designed for 1,000 visitors a day and as a shelter for up to 3,800 people in an emergency."Helsinki stands on stable ground; they have a good foundation," says Jaaksi. "The city is overcrowded, and we have long and cold winters here. The underground offers us more space and protects us from bad weather," agrees Ilkka Vähäaho, director of the Helsinki Geotechnical Institute. Vähäaho also talks about other reasons to expand the city underground, including: "Finland needs open space even in the city centre: building part of the city underground creates more space on the surface."

That is why Helsinki's plan could bring a new approach to living underground. With up to 60% of the population expected to live in cities by 2050, homes will need to be found for 2.5 billion people. City land is finite, and urban space is a limited resource. There is a practical limit to how tall buildings can be, and in many cities, such as Paris, Mexico City and Singapore, urban layouts, listed buildings and green spaces suggest that the answer does not lie in more skyscrapers. So why not build underground?

In 2017 Paris launched a competition called Reinvent Paris 2.

It invited designers to make use of unused and under-used spaces owned by the city, most of them underground: the cellars of historic buildings, tunnels along the Seine closed to traffic, disused reservoirs, old car parks and tarmac surfaces. These spaces were turned into restaurants, shops and micro-farms for edible insects.

Architects in Mexico City took a different approach

They designed a 300-metre-deep pyramid called the Earthscraper, to be built on the main square of Mexico City. But its price tag of $800 million (€726 million) put an end to the plans.

Meanwhile, the government in Singapore has invested

...more than $188 million (€170 million) in engineering and research for underground construction. It has also changed the law so that all basements now automatically belong to the state. According to Singapore's statistics office, its 5.53 million people share just under 719 square kilometres of land, making it the third most densely populated place on Earth. So far the city has built upwards, with buildings of up to 70 storeys pressing towards the island's coastline. With an estimated 1.5 million more people expected in the next 15 years, Singapore's options are limited by its size. The city is exploring the idea of an Underground Research Park 80 metres below the surface, which would house 4,500 scientists and researchers in an underground skyscraper with shopping centres, green infrastructure, motorways, trains and metro lines, and ventilation shafts. Its cylindrical form is designed to withstand earthquakes. But building costs twice those of a skyscraper have dampened hopes that it will be built.

"Global warming, as in Singapore with its difficult weather and fear of Chinese aggression, is likely to drive cities underground. The result will be cities built partly underground to limit the effects of extreme weather, and partly above ground to ensure enough natural light."

In the past few years it has become clear that the Singapore government has started moving strategic facilities underground.

For example, the Singapore Caverns under the city, once used to store ammunition, and the Jurong Rock Caverns under Singapore's Jurong Island are being used as the first underground oil storage facilities. Built by Hyundai Engineering and Construction, they have five large caverns 100 metres deep and 8 kilometres of tunnels. In March the government unveiled the first stage of its Underground Land Use Plan in three districts: Marina Bay, Jurong Innovation District and Punggol Digital District. At a media briefing, Singapore's chief planner Hwang Yu-Ning said the plan would set aside space for the future, create capacity for growth and protect the environment. She highlighted Singapore's first 230 kV underground substation, in the basement of a commercial building, and the district cooling of buildings along the Marina Bay waterfront, which cuts the energy used for cooling by 40%. Water cooling has cut CO2 emissions by 34,500 tonnes, equivalent to about 10,000 cars on the road.

Storing energy underground

This is one of the biggest draws for urban planners, according to Dale Russell, Professor of Innovation Design Engineering at the Royal College of Art. "Building underground saves space and certainly energy," Russell explains. "The topography itself can produce energy: rocks absorb heat in summer, cooling the city, and in winter they release it like large radiators to heat the buildings. By 2069 we can expect fully self-sufficient transport and ecological systems underground, as well as hydroponic farming under artificial light to supply the city with its own food."The subsoil under Paris, for example, is very humid and has a constant temperature of around 14°C, whatever the temperature outside. "If we build somewhere very hot, like Dubai, or very cold, like Scandinavia, the building will cost less over the long term. The temperature is stable, so less cooling and heating is needed," says Gunnar Jenssen, head of underground psychology at the Scandinavian research organisation SINTEF.

The problem, says Asmo Jaaksi, chief architect of Helsinki's underground Amos Rex museum, is not the building itself. "It's about making people feel good when they are underground," he explains. "The museum had run out of space and needed a new, larger building. At the modernist Rex cinema, the only option was to dig out the square in front of it and 'sink' the museum below. We found that people need contact with the surface."Jaaksi's answer was sculptural light shafts that let in daylight and offer intriguing views into the building. In London, the world's first underground farm, Growing Underground, was set up in a former air-raid shelter beneath Clapham Common. It grows lettuce, garlic, wasabi mustard, fennel and radishes hydroponically and sells them to M&S and Ocado. The plants grow under pink light, which becomes uncomfortable after a while.

The Lowline Lab in New York

ran an experimental project from October 2015 to March 2017, growing plants underground with solar collectors and copper pipes that brought natural light into the tunnels below. It successfully grew more than 100 different kinds of plant. The Lowline Lab is preparing the first green underground space for 2021.

Will we ever join the people of Cappadocia and move below the Earth's surface?

Jaaksi believes so. "Life underground will be part of the future of cities," he says. "Underground spaces are temperate, with smaller swings in temperature; they retain heat, and they offer better drinking-water reservoirs, less contaminated by surface water, as well as geothermal energy. As global warming makes the surface hostile, underground may be the safest place to be. Do you really want to be in a skyscraper when a tornado hits?"