In 1963, a resident of Derinkuyu, in the Cappadocia region of Turkey, knocked down an interior wall while renovating his house. Behind it lay an unexpected void which led to an astonishing discovery: an entire underground city that, at its peak millennia ago, sheltered up to 20,000 people across depths reaching 85 metres.
Little is known about its original builders or their reasons for excavating it, whether as a climate refuge or a stronghold against invading armies. What is clear is that, once again in human history, the depths of the earth became a vital space for living, shelter, and storage.
Today, major underground infrastructure continues to push the boundaries of our cities. Rail and road tunnel networks speed up transport, link distant points, and connect communities.
Given the challenges of deep tunnelling, these massive schemes demand engineering capabilities well beyond the reach of most contractors. This is where ACCIONA, drawing on nearly a century of experience, has established itself as an international benchmark capable of delivering every phase of a project's lifecycle, from financing through to tunnelling, construction, and asset maintenance. In addition, the company has built a strong track record in positive social impact and sustainable practices.
This feature article takes a journey deep underground, where ACCIONA has excavated more than 800 kilometres of tunnels throughout its history. Join us to explore some of the greatest challenges in building the subterranean cities of our time.
ACCIONA has excavated more than 800 kilometres throughout its history.
In historical scholarship, an event is generally considered to pass from the contemporary era into history once 50 years have elapsed. By that measure, ACCIONA marked a historic milestone in 1973 when it deployed its first Robbins tunnel boring machine on the Tagus-Segura Aqueduct in Cuenca, in central Spain, more than half a century ago.
Looking back to its earliest tunnelling works requires going even further into the past, to the mining infrastructure undertaken in 1953 by Entrecanales y Távora, the origin of modern ACCIONA. Since then, its operations have expanded worldwide, delivering rail and road tunnels across Spain, Italy, Brazil, Canada, Norway, Ecuador, and Australia, among others.
Mobilising the human, financial, and logistical resources needed for major rail and metro schemes is such a vast undertaking that rarely can a single contractor manage every responsibility. However, the company provides bespoke financing vehicles, deep expertise in major civil works and urban developments, as well as the capability to handle network operations and maintenance.
The most compelling demonstration of this end-to-end capability is São Paulo Metro Line 6, the largest infrastructure project underway in Latin America. Here, ACCIONA oversees the turnkey delivery of a 15-kilometre underground alignment featuring 15 stations, representing an investment exceeding €3 billion.
Every metro or rail construction project begins with geotechnical investigations to understand the ground conditions along the alignment. This relies primarily on core drillings, sinking small-diameter boreholes from the surface down to tunnel depth. The goal is to build an accurate geological model of the expected strata before site works begin. This phase guides key excavation decisions around project ground conditions, helping to reduce contractor risk by providing a firm baseline against which to evaluate budgets.
Site reconnaissance does not end there, as the tunnel boring machines themselves perform horizontal probe drilling and ground-penetrating radar analysis as they advance. In recent years, ACCIONA has also applied artificial intelligence techniques to anticipate ground behavior in upcoming sections.
TBMs are not used on every drive. Depending on the complexity and scope of the scheme, teams may opt for conventional tunnelling methods using excavators and controlled blasting, or adopt hybrid approaches that deploy boring machines only on specific sections.
Much like deep-sea trenches or outer space, working underground means facing harsh environments with severe physical constraints. When it comes to kilometre-long tunnels through challenging ground, only purpose-built tunnel boring machines can meet the demand. Weighing upwards of 4,000 tonnes, these mechanical titans come in a variety of configurations and shield types.
Certain machines are built to bore through hard rock. These open and single-shield TBM models were deployed on the Guadarrama, Pajares, and Follo Line projects. In other settings, excavation must push through water-bearing silts and soft clays. Here, teams turn to Earth Pressure Balance (EPB) and Slurry Mixshield machines, which rely on a pressurised face chamber to balance ground pressures and prevent surface settlement. Similar setups were used on underground networks in Quito, Sydney, and São Paulo, among others.
For kilometre-long drives through demanding ground, only tunnel boring machines are built to meet the challenge. Weighing upwards of 4,000 tonnes, these underground titans are tailored with specialised shields and custom cutterhead designs.
Heavy machinery is only half the story; the people running it are just as critical. Few operators bear as much direct responsibility as the crews piloting these tunnel boring machines. While airline pilots look after the safety of their passengers, the specialist at the controls of a TBM cabin, monitoring banks of diagnostic displays on 24-hour rotating shifts, must safeguard buildings on the surface while steering a multi-million-euro asset through unpredictable ground. They are the frontline aces of civil engineering, and their craft takes years of on-site experience to master.
The sector is also seeing a steady influx of women into these positions, driven by initiatives ACCIONA has introduced across its Australian portfolio, such as the Western Harbour Tunnel. There, female trainees are being coached as shift supervisors and TBM operators, preparing them to take on these major site roles.
The shear scale of heavy tunnelling often stands in sharp contrast to the delicate touch the work requires. On the Bologna metro, for example, ACCIONA mapped centuries-old Renaissance buildings to prevent vibration damage during tunnel drives.
Beyond initial site surveys, engineers use specialist methods to protect structures on the surface. One key solution is compensation grouting.
From shafts excavated clear of the tunnel trace and the building footprint, horizontal tubes are drilled directly beneath the foundations to pump cement grout under high pressure.
With every forward drive of the TBM, concurrent grout injections offset ground movement as it happens, ensuring structural stability overhead. This method safeguarded the streets above the Metrotrén tunnel in Gijón—acting almost like a protective cradle beneath the buildings.
On the Bologna metro, for example, we had to survey and map nearby historic buildings, many of them Renaissance landmarks, to protect them from vibration impacts as the tunnels advanced.
Protecting standing buildings is only part of the equation. Major civil works frequently run into valuable archaeological remains, as was the case at the Cortijo Lobato photovoltaic plant in Extremadura.
During underground tunnelling, that probability multiplies significantly. On the rail link to the Warner Bros theme park in Madrid, excavations uncovered 3,000 Visigothic graves, which were fully protected in direct consultation with heritage authorities.
Excavated finds can also be incorporated straight into transport infrastructure. Along Line 11 of the Madrid Metro, an ancient timber irrigation channel discovered during construction was restored and placed on permanent display inside the station.
In other instances, heritage assets are embedded in the urban fabric itself. During construction of the Quito Metro, teams carefully dismantled the central Plaza de San Francisco stone by stone, cataloguing every piece before rebuilding the colonial square once station works below were complete.
ACCIONA builds sustainability into its infrastructure projects right from the tender stage and carries it through final delivery. Finding ways to reduce embodied carbon and limit surface disruption receives the same technical scrutiny as structural load calculations and cost management. Rail and metro networks are no exception.
To begin with, TBMs run on electric drives to eliminate emissions inside the tunnel bore. Electrification extends above ground to site machinery, such as the electric wheel loaders working at the Line 11 precast segment plant in Madrid. Where combustion engines remain necessary, sites use renewable Hydrotreated Vegetable Oil (HVO), a biofuel produced from waste vegetable oils that cuts life-cycle CO₂ emissions relative to standard fossil fuels.
The company’s water treatment capabilities are also put to work on tunnelling sites, using industrial treatment units to clarify groundwater inflows.
Circular economy principles are central to spoil management, diverting excavated material to create public parks, raise building pads for housing, or build sub-bases for local roading. On the Follo Line outside Oslo, 90% of the crushed rock extracted by the TBMs was placed nearby to create a level development foundation for a new residential suburb. Moving that spoil via continuous overland conveyors removed thousands of heavy truck movements, eliminating more than 27,000 tonnes of vehicle-related CO₂ emissions.
ACCIONA weaves sustainability into its infrastructure projects from the very first bid, carrying it through every phase of construction. [...] Such was the case on the Follo Line project in Oslo, where 90% of the TBM spoil was deposited within the project footprint and repurposed into an embankment supporting a new residential community.
Truly sustainable and resilient infrastructure must improve daily life, both during decades of operation and throughout the disruption of construction. This requires engaging local communities from the earliest planning stages. Beyond installing acoustic and dust enclosures on urban sites, as was done in Australia, teams appoint dedicated Community Stakeholder Managers to listen to residents and resolve local issues directly.
Turning major construction sites into hubs for technical training and skills development is another key priority. Whether through programmes supporting female site teams at the São Paulo Metro Line 6 precast segment factory, or by upskilling and hiring local workers so they retain transferable trade skills long after the civil works conclude, these projects leave a lasting workforce legacy.
Much of the heavy engineering that goes into rail tunnelling remains out of sight, both during construction and throughout daily operations. Yet these underground corridors serve as the vital arteries of major cities.
Moving people and vehicles is a matter of transit statistics, but behind the figures are real stories of human connection and fresh opportunity. In São Paulo, Line 6 will provide residents in disadvantaged neighbourhoods with a direct rail connection to the city’s financial heart, bringing access to work, education, and public services within reach.
No engineering project, however impressive its machinery or complex the hurdles overcome, fulfills its purpose unless it tangibly improves everyday lives. Schemes like Line 6 in Brazil, the Quito Metro in Ecuador, and the M-30 Tunnels in Madrid stand out among the most transformative infrastructure assets worldwide. Reshaping the world to make it more liveable for future generations remains the core mission of ACCIONA.