María Gómez Bravo | Tungsteno

In their search for more space, cities first reached skyward. Skyscrapers and landmark towers reshaped skylines and often became symbols of major capitals. Yet a quieter revolution was also taking place underground. It began with a practical aim: moving the infrastructure that keeps a city running beneath its streets. First came sewers and water mains, followed by railways, subway lines, utility cables, parking garages, and even roads.

As land at the surface becomes scarcer, the space below is increasingly valuable. Every new tunnel, cable or facility must fit among the infrastructure already there without blocking what future projects may require. Yet pressure on surface land is only part of the story. Geology, climate, and other local conditions also shape the way cities develop underground.

A City Carved Out of Rock

The granite bedrock beneath much of Helsinki has helped shape the city’s underground development. For decades, it has been excavated to create tunnels, parking garages, energy facilities, and sports venues. Helsinki has made systematic use of its distinctive geology since the 1960s. By the time it began drafting its first dedicated underground plan, around nine million cubic meters of rock had already been excavated for some 400 facilities and tunnels. 

Many of these spaces have a dual role. In everyday life, underground facilities house swimming pools, sports centers, and other recreational spaces carved out of the bedrock. In an emergency, they can also serve as civil defense shelters. Shared tunnels carry district-heating and cooling pipes, high-voltage power cables, and telecommunications networks, which together extended for around 300 kilometers when the plan was drawn up.

The underground swimming pool in Itäkeskus is located in bedrock 50 meters below the surface, inside one of Helsinki’s civil defense shelters that has been adapted for everyday use. Credit: ALESSANDRO RAMPAZZO/AFP via Getty Images.

As demand for underground space grew, Helsinki had to consider not only where to build new facilities, but also which parts of the bedrock should be left untouched for future use. The result was the Underground Resources and Master Plan, which mapped existing and planned caverns, facilities, and tunnels, while reserving space for long-term projects. The first plan identified 40 new areas as rock reserves and more than 100 sites for future construction. Ilkka Vähäaho, who headed Helsinki’s geotechnical division for many years, has explained that the rapid expansion of underground construction since the beginning of this century has made it increasingly necessary to manage where excavation takes place.

Helsinki’s first underground plan put it simply: bedrock can be excavated only once. Buildings can be replaced and plots of land repurposed, but caverns carved into the rock are permanent and may obstruct future infrastructure. Helsinki therefore began planning not only what to build underground, but also the empty space that would need to remain available.

A Shield Against Winter

In Montreal, Canada, winter shapes not only how people get around, but also how they use the city. Beneath its streets, a network of pedestrian walkways links offices, universities, hotels, shopping centers, and other buildings without requiring people to step outside. Used by about 500,000 people a day, this underground city allows residents and visitors to travel across much of downtown sheltered from the cold.

Yet the nearly 32 kilometers of walkways that make up RÉSO, Montreal’s underground pedestrian network, were not originally built as a response to the climate. The network began to take shape around Place Ville Marie between 1962 and 1967, expanding alongside major downtown property developments and the growing metro system, particularly in the years leading up to Expo 67.

The Montreal Trust shopping center is part of RÉSO, the underground network that connects buildings, stores, hotels, and metro stations in downtown Montreal via nearly 32 kilometers of walkways. Credit: Pascal QUITTEMELLE/Gamma-Rapho via Getty Images.

There was no single master plan. Instead, connections were added over the decades as new buildings were incorporated into the system. This organic growth turned this underground network into an extraordinary asset for both the downtown economy and the everyday mobility of its residents during the coldest months. Today, RÉSO is so woven into the life of the city that it is hard to separate its economic role from its practical value as a way of getting around.

Cold weather does affect how people move around Montreal. A study by Luis F. Miranda-Moreno and Aleksiina Lahti found that outdoor pedestrian traffic declines in winter, with temperature, humidity, wind, and precipitation all influencing the decision to walk. Yet RÉSO has become so embedded in the workings of downtown that the weather outside makes surprisingly little difference to how residents use it. 

An analysis by Professor John Zacharias on underground pedestrian trips in the city found that weather has no impact on the distance people walked or the proportion of the journey spent underground. The starting point mattered far more. What began as a series of separate connections has become part of the everyday experience of crossing downtown Montreal, regardless of the weather.

150 Meters Beneath Singapore

Beneath Jurong Island, off the coast of Singapore, five enormous caverns lie hidden about 150 meters below ground. The Jurong Rock Caverns can store up to 1.47 million cubic meters of liquid hydrocarbons for Singapore’s energy and chemical industries. Building them freed about 60 hectares at the surface for other industrial activities. In a country with a land area of just over 730 square kilometers, that is a significant gain.

For decades, Singapore has been waging a battle to create room in a city-state with little land to spare. It has reclaimed land from the sea, built upward, and increasingly looked below ground. Mining engineer Yingxin Zhou captured the idea vividly at the 2018 World Urban Forum: “Singapore doesn’t have gold, oil, or diamonds. But we mine something equally precious: space.”

In total, Singapore already has about 300 kilometers of tunnels and plans to add a further 60 kilometers over the next decade, according to the Master Plan 2025. Credit: John Seaton Callahan.

The Jurong Rock Caverns are perhaps the most spectacular manifestation of Singapore’s search for underground space, but much of this transformation remains out of sight. On the main island, the underground levels closest to the surface accommodate shops, parking garages, services, and pedestrian links. Deeper down are rail lines, roads, and networks for water, sewer, and electricity. In total, Singapore already has about 300 kilometers of tunnels and plans to add a further 60 kilometers over the next decade, according to the Master Plan 2025.

As Singapore has made increasing use of its subsurface, long-term planning has become necessary. In 2007, the government established a dedicated inter-agency task force. Three years later, underground development was incorporated into the country’s economic strategy, as Zhou and Jian Zhao describe in a study published in Tunnelling and Underground Space Technology. The Master Plan 2025 now explores new possibilities, from storage caverns and utility tunnels to energy infrastructure, while reserving room for future needs.

Creating space at the surface can also lead to costs underground. Subterranean spaces require lighting, ventilation, and often dehumidification. A study published in Sustainable Cities and Society also identified potential benefits associated with transportation, more compact land use, infrastructure, and geothermal energy. Depth alone does not make a project sustainable. What matters is what is moved underground, how much room that creates above ground, and how long the space can be put to good use.

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MARÍA GÓMEZ BRAVO | Tungsteno

In March 2001, television viewers around the world watched the destruction of the Afghan Buddhas of Bamiyan. For weeks, explosions ordered by the Taliban regime gradually reduced the two ancient sculptures to rubble. Twenty-five years later, those images have become a symbol of the deliberate destruction of cultural heritage and continue to fuel the debate about what happens when monumental works—designed to last for centuries—disappear.

Long before Bamiyan, earthquakes, fires, wars, the passage of time, and the reuse of their materials caused some of the colossal monuments of antiquity to vanish. Of many of these magnificent sculptures, only traces remain: the texts that described them, the sites where they once stood, scattered fragments, or the imprints they left on the landscape. Today, these traces are complemented by new tools that allow us to reconstruct and study monuments that no longer exist physically.

A Colossus in Constant Reconstruction

Few sculptures have inspired as much literature while leaving us with so little certainty as the Colossus of Rhodes. The image of the giant towering over the entrance to the harbor is part of the collective imagination, although we do not know whether it actually stood there. This interpretation emerged centuries after the bronze statue disappeared and ultimately shaped the way we still imagine the colossal figure dedicated to the god Helios today.

Today we know that the Colossus was erected between 295 and 283 BC to commemorate Rhodes’ victory following the siege led by Demetrius Poliorcetes and that it stood approximately 34 meters tall. Nathan Badoud, author of The Colossus of Rhodes: Archeology of a Lost Wonder, suggests that it may have been located near the present-day Palace of the Grand Master. There is also broad consensus on the cause of its destruction: an earthquake toppled it barely half a century after its construction.

Beyond these facts, much remains uncertain. No identified fragments of the statue have survived, and ancient sources offer only sparse descriptions. Archaeologists and historians have reviewed the topography of ancient Rhodes, classical texts, and the political context in which it was erected to try to determine where it stood, what it looked like, and what it meant to a city that had by then become one of the great powers of the eastern Mediterranean.

Research drawing on classical literature, combined with disciplines such as topography and engineering, challenges the image and location traditionally associated with the Colossus. Photo: Culture Club/Getty Images.

Engineering also plays an important role in this research. Knowledge of Hellenistic metallurgy, materials, and the structural behavior of bronze makes it possible to assess the feasibility of the various hypotheses regarding its construction. Ancient sources, including the texts of Pliny the Elder, remain fundamental, but they are no longer the only tools available for attempting to reconstruct a colossus that disappeared more than two thousand years ago.

Reconstructing from the Original Site

We do not know for certain when or how the Statue of Zeus at Olympia disappeared. We do know that around 430 BC, a monumental figure of Zeus presided over the temple dedicated to him at the Greek sanctuary in the Peloponnese. The building had been erected a few decades earlier, in the first half of the 5th century BC, and its pediments and metopes were decorated with an extensive sculptural ensemble. Opposite the temple, outside the sacred precinct of the Altis, stood the workshop where Phidias worked—the sculptor responsible for the monumental depiction of Zeus.

Excavations at the site uncovered terracotta molds, ivory fragments, and materials used for glassworking. A small cup bearing the inscription Pheidiou eimi (“I belong to Phidias”) was also found. The chronology of the finds dates activity in the building to the third quarter of the 5th century BC, coinciding with the period when the sculpture—one of the Seven Wonders of the Ancient World—was constructed.

The materials used in Phidias’s great sculpture of Zeus and the methods used to make some of its components are known from molds found at his workshop,outside the sacred precinct of the Altis. Credit: Hulton Archive/Getty Images.

The Statue of Zeus was a chryselephantine sculpture of extraordinary dimensions. Art historian Judith M. Barringer estimates its height at around 13.5 meters, a scale that required the figure to be constructed using an internal framework—likely made of wood—over which various materials were applied, primarily ivory for the skin and gold for the garments. The molds found in the workshop allow us to study how some of these components were made, although we still do not know precisely how the entire figure was assembled.

The combination of materials also affected its preservation. Ivory and wood are sensitive to changes in humidity. When the Greek traveler and geographer Pausanias visited Olympia in the 2nd century CE, he noted the presence of olive oil in a depression in front of the statue. This has been interpreted as a possible measure to protect the ivory from environmental conditions, although later research also suggests an optical function: the surface may have helped reflect light onto the figure. Both interpretations remain possible.

The Images That Survived the Buddhas

After the explosions, the two large niches at Bamiyan were left virtually empty, and the remains of the sculptures lay scattered at their feet. To study them, researchers had access to something that never existed in the case of the Colossus of Rhodes or the Statue of Zeus at Olympia: photographs taken before their destruction.

The world’s tallest standing Buddha statue, located in the province of Bamiyan, was destroyed by explosions ordered by the Taliban regime in 2001. Credit: JEAN-CLAUDE CHAPON/AFP/GETTY.

Just three years later, Armin Grün, Fabio Remondino, and Li Zhang of ETH Zurich drew on three collections of images from before 2001 to digitally reconstruct the Great Buddha. The study, published in The Photogrammetric Record, combined photographs taken by visitors with others taken specifically for measurement purposes. Photogrammetry made it possible to reconstruct its shape and dimensions and generate a three-dimensional model.

Digital techniques were also applied to the cliff face. Laser scanning and photogrammetry were used to document the niches and remains and obtain an accurate record of their condition. Years later, some projects incorporated these data into virtual environments to study different conservation and restoration measures before undertaking any work on the site.

In 2017, a study published in PLOS ONE analyzed the layers of paint adhering to some fragments to reconstruct part of the history of their decoration. The analyses identified egg proteins used as a binder in the original layers and cow’s and goat’s milk in later repaintings, making it possible to distinguish between different stages and materials used on the Buddhas over time.

Reconstructing or Preserving the Absence

Digital technologies have made it possible to reconstruct the appearance of the Buddhas with a precision that was unthinkable a few decades ago. But turning those models into a physical reconstruction raises a different debate. Twenty-five years after their destruction, the future of the Bamiyan niches remains unresolved.

Proposals have ranged from leaving them empty to partially or fully reconstructing some of the figures. UNESCO and experts who have worked on the site have spent years analyzing the various possibilities. It is not merely a matter of determining what can be reconstructed: it is necessary to assess the stability of the cliff, the original fragments that could be reused, the authenticity of any potential intervention, and its significance for the communities connected to the valley.

The possible reconstruction of the Bamiyan Buddhas, destroyed by the Taliban regime in 2001, has kept alive the debate about how much of their absence should be preserved. Credit: KAMRAN SHAFAYEE/AFP via Getty Images.

Added to these questions is the historical value of the destruction itself. The 2001 explosions forever altered the site, and their traces are now part of Bamiyan’s recent history. For this reason, one proposal is to leave at least one of the niches empty as a testament to what happened, while others advocate restoring, to varying degrees, the presence of the Buddhas.

Tungsteno is a journalistic laboratory that explores the essence of innovation.