I want you to think of an example of underground infrastructure.

No matter your background or expertise, it’s likely that your mind immediately went to something that provides a critical service to those of us above ground. Perhaps you thought of a rail or road tunnel. Perhaps a network of water and waste pipes, or dense bundles of electricity or telecommunications cables. While this picture of the underground as a service layer for an overlying urban form will feel familiar to many, in reality it belies much of what that realm has meant to us throughout our history.

For example, did you know that the festival of Halloween traces its routes to a singular cavern in Ireland? Or that the Moscow Metro was a key propaganda tool in the 1930s? What about the permafrost-clad facility that is home to more than a million crop samples?

Reaching beyond the obvious to those surprising and lesser-told stories of the subterranean may yet provide the inspiration and ideas we seek for a more underground future.

“An increasing proportion of facilities
will move underground, to preserve
‘topside’ space for housing and ecology.”

From ground level, Oweynagat is unassuming. Nestled near the edge of a field in a rural part of northwest Ireland, Oweynagat is part of what was once an extensive royal settlement – the hub of the ancient Irish kingdom of Connaught, dated at 5,500 years. Legend has it that it was home to an immortal queen called Morrigan and her three wildcats, which gives the cave its name, Uaimh na nGat (Cave of the Cats).

Sheltered and subterranean environments have long been safe places for humanity to explore its creativity. Ancient imagery adorns walls at numerous sites across the world – everywhere from Australia to South Africa, and Indonesia to South America. They are a reminder that we have always left our mark on our surroundings. The Cueva de Ardales – a cave in southern Spain – is home to the earliest example of rock art so far discovered. The site contains over 1,000 prehistoric paintings and engravings. One set of images, consisting of dots, finger prints, and hand-stencils, all created with red ochre pigment, is especially important. It has been dated to at least 64,000 years ago, which suggests the images were created by Neanderthals. In 2022, researchers showed that the same cave was used as a canvas for artwork and as a burial place for more than 50,000 years.

Playgrounds for design and architecture

Underground spaces are still widely used by artists from all backgrounds – contemporary examples include Ra Pualette’s hand-carved sandstone cave, Herbert Baglione’s haunting ‘shadow art’ in abandoned bunkers, and iconic graffiti-filled tunnels like Leake Street in London, Tunnel des Tuileries in Paris, and Freedom Tunnel in NYC. Museums, dance schools, and recording studios also increasingly make use of subterranean spaces.

Metro stations have become another playground for design and architecture. In their book Underground Spaces Unveiled, authors Han Admiraal and Antonia Cornaro write about the opening of the Moscow Metro in 1935. The grand designs of the stations were “a socialist reaction to the dull, drab and uniform systems of the West.” With high-ceilinged halls, ornate lights, and floors covered with reflective marble, they served “a propaganda purpose in their explicit grandeur, creating an atmosphere that would negate any negative feelings of being underground.” In addition, the distinctive appearance of each station acted as its own wayfinding tool, which meant that passengers unable to read a station’s name could still recognize it from inside the metro car. That point inspired two Swedish artists to successfully campaign for the introduction of art onto the Stockholm Metro in the 1950s. Today, more than 90 of the network’s 110 stations feature artworks, with each station retaining its own distinct design. It is now considered the world’s longest art gallery.

For almost 15 years, international teams of scientists have carried out experiments on the LHC to probe the nature of our universe by studying its fundamental building blocks. Scientists searching for neutrinos also have to travel deep underground to find answers. These ghostlike, subatomic particles ejected by our sun (and other stars) barely interact with ordinary materials. Catching one involves constructing huge tanks filled with a fluid – often a dense form of water – in subterranean caverns. Some detector tanks can hold the same amount of water as twenty Olympic swimming pools. Their size increases the tiny probability of the elusive neutrino making an appearance. And by doing this research underground, scientists make use of the overlying rock to ‘filter out’ the other particles that continuously rain down on our planet.

Believe it or not, botanists and agronomists also work in subterranean facilities. Rather than hunt for particles, these scientists are safeguarding the genetic diversity of the world’s crop supply, in the face of various threats. Described as “Noah’s Ark for seeds” and currently home to 1.2 million samples, the Global Seed Vault can be found 152 meters inside a mountain on Svalbard in Norway, known as Spitsbergen, in the Arctic Circle. Its remote location was chosen because it offered “almost perfect conditions” for long-term, secure seed storage. The presence of permafrost provides temperatures of -5 °C, which means that the seeds, sourced from a network of gene banks all over the world, will stay frozen, even in the event of a power cut. In addition, cities like Taiwan, Seoul, and London are transforming vacant metro tunnels into vertical farms, where they grow salad leaves and microgreens to sell locally.

Vertical farming has the added bonus of bringing greenery into underground spaces. For Lee, “such a measure will promote the physical and psychological well -being of the underground community.”

Projects like these push the boundaries of tunnelling in numerous ways. For a start, there are no standard rules or guidance for the design of underground research facilities. Each one is bespoke, with its own unique set of requirements. The same is true for monitoring and maintaining the facility once it has opened. To meet this need, CERN has developed its own robots that can capture data and photos from the tunnels, and an algorithm that identifies cracks and other features in real-time. They’re also exploring the use of fiber-optic cables to remotely measure underground movements. This would allow them to continuously monitor the tunnels, even when an experiment is underway. These tools look set to find widespread use in other tunnel projects.

CERN’s scientists hope to add another underground facility to the site in the coming decades, which will house the next generation of particle collider. As well as pushing the boundaries of science, this project would involve the construction of a 100 km-long tunnel. That’s almost twice the length of the Gotthard Base Tunnel, which currently holds the title of ‘world’s longest tunnel.’

“Bringing greenery into
underground spaces will
promote the physical
and psychological
well-being of the
underground community.”

The region’s geology goes some way to explaining why it is also the site of a large number of ‘underground cities,’ including the most sophisticated, Derinkuyu. The city’s network of tunnels and caverns, carved into the soft rock, acted as a stronghold in times of war and other crises, and as an everyday storage facility and transport link. Each of its exits was marked with a massive stone door that, when needed, could be rolled into place from the inside of the tunnel. The presence of extensive ventilation shafts, wells, and rainwater tanks suggests that the city was designed to be self-sufficient; a characteristic that was put to great use during the Arab–Byzantine wars. Excavated to a depth of 85 meters, and split over eight levels, Derinkuyu was large enough to have temporarily housed 20,000 people along with their livestock.

“Creating distinguishing features that
provide visible cues and add to the enjoyment
of underground spaces should be used,
as opposed to uniform
designs of stations along a route.”

It’s possible that Derinkuyu was a source of inspiration for another underground metropolis, this one built in anticipation of a nuclear war that (thankfully) never happened. Between 1969 and 1979, a vast network of tunnels and bunkers – covering an area of 85 km2 – was constructed beneath Beijing’s city center. Called simply the ‘underground city’ (Dìxià Chéng), it was equipped with a wide range of facilities, including restaurants and factories. It had an elaborate ventilation system too, as well as gas- and water-proof hatches. According to one account, the city was designed to safely house half of Beijing’s population for four months. Though Dìxià Chéng was never used for its intended purpose, parts of it went on to become low-cost housing and office facilities. In 2017, National Geographic reported that more than a million people were living in these former bunkers.

While some underground cities were purpose-built, there are also countless examples across the world of existing subterranean structures being repurposed into places of refuge. They include Naours, a chalk mine in northern France, that sheltered people fleeing conflicts from the Middle Ages through to the 17th century. The London Underground tunnels provided vital protection to city- dwellers during the first and second World Wars. And even today, residents in Coober Pedy – an opal mining town in the center of the South Australian desert – live in underground homes in disused parts of the mines.

On first glance, this view of subterranean spaces as a safe haven seems somewhat at odds with Lee’s fourth and final psychosocial factor, “perceived security.” But as she explains, security has a very specific meaning in this context; “it refers to risk or dangers stemming from human behavior (such as terrorist attacks, crimes, etc.).” She also says that a lack of landmarks “may provide an opportunity for those with a criminal intent.” In order to avoid these associations, she urges designers to think carefully about visibility, lighting, wayfinding, and surveillance in underground environments.

is a physicist and science writer. Winkless communicates scientific knowledge to a wider audience in a seamless style. She describes herself as a “tunnelling nerd” and contributes regularly to international journals. Her recent books are "Science and the City" and "Sticky".