Like mythological behemoths bridging distant worlds, these machines can bite through the core of a mountain or chew a path beneath the seabed. They are staggering in scale, reaching lengths of 120 metres and diameters of nearly 18 metres—roughly the height of a five-storey building. Tipping the scales at 5,000 tonnes (about the same as 320 buses) and packing 5,600 kW of power, machines like the S-880 Qin Liangyu—built to bore a subsea road tunnel in Hong Kong—are giants of modern industry.

Mariano Calero, a mechanical engineer and Tunnel Manager at ACCIONA, describes them as “mobile factories.” To help us get our heads around the scale, he compares the mechanism to a standard DIY drill: “You need the cutting wheel to spin on its axis while pushing against the rock face, just as you’d lean into a drill to make a hole in a wall. In a TBM [Tunnel Boring Machine], we achieve this using hydraulic cylinders,” he explains.

But there is little point in piercing a mountain without shoring up the hole as the machine’s head advances. “TBMs dig and install the lining in one near-continuous motion, with barely 10 metres between the two to support the ground. This is usually done with reinforced concrete up to 60 cm thick,” says Antonio de Barba, a mining engineer at ACCIONA. It sounds simple enough, but the real trick is cutting without breaking. The machine must be tougher than the rock itself while withstanding friction-induced heat that can reach hundreds of degrees. 

Like any titan, these machines have an Achilles' heel: the wear and tear on the cutting wheel. “As they move forward, the tools on the head effectively 'scratch' the ground. They need incredible hardness and the 'grit' not to snap, as well as being good thermal conductors to bleed off the heat from the friction,” explains Sandra Tarancón, a researcher at the Polytechnic University of Madrid.

These “teeth”—each as large as a man's torso—are made from a metal matrix embedded with ceramic particles of tungsten carbide, providing the hardness required to bite into the earth. According to the expert, the metal used is typically cobalt, which provides the necessary resistance to both wear and high temperatures.

When asked about the heavy hitters of the industry, De Barba points to the aforementioned Qin Liangyu, named after a famous Ming Dynasty general, and Bertha—named after Seattle’s first female mayor, Bertha Knight Landes. “Bertha was one of the largest ever built, famous for tackling the treacherous ground beneath the city of Seattle,” he says. There is also Linghang, currently chewing through the earth beneath the Yangtze River in China with a 15.4-metre diameter head.

Spain is also a major player, with machines like Dulcinea—named after the heroine of Don Quixote. With a 15.2-metre diameter, she cleared the path for Madrid’s M-30. “After 16 years 'gathering dust' at our facilities, she was refurbished and upgraded for a major project: a 2.3-kilometre twin-tube tunnel in Poland,” De Barba notes.

The human teams behind these feats often form a close attachment to these machines; after all, they are their primary companions for months or even years at a time. “Miners like to name them, and even bless them before they start. We look after our machines because if you take care of her, she’ll take care of you,” De Barba admits.

“There are women who leave a mark, and TBMs want to bear their names” was the slogan for the Lima Metro extension project. It invited citizens to choose the names of national heroines, resulting in Delia—honouring Delia Tasaico, the first Peruvian woman to graduate as a mining engineer—and Micaela, named after independence leader Micaela Bastidas.

In Sydney Harbour, Australia, work is nearing completion on the second stage of the Western Harbour Tunnel using two 16-metre diameter ACCIONA machines. “As a new feature, it incorporates a robotic arm to change the cutting discs automatically when they wear out, removing the need for humans to enter the head to do it manually,” says Calero.

However, when it comes to “joining worlds,” the ultimate prize is connecting Africa and Europe. Our experts agree that the “holy grail” would be a tunnel under the Strait of Gibraltar. “The project has unique complexities, such as its extreme depth and a far-from-easy geology,” De Barba notes. Not to mention the crushing pressures of up to 17 or 20 bars.

Tarancón adds that while many ambitious projects exist, subsea tunnels remain the most difficult because they are the least explored. “In the case of the Strait, the lining would need to be perfectly sealed with waterproofed concrete and be entirely airtight to prevent a catastrophic collapse under the pressure.”

“It’s an idea that has been on the drawing board for decades,” Calero concludes. “With current technology, we can't quite do it yet. But future advances will allow it to happen, sooner or later.”

Source:

• Tarancón Román, Sandra (2025). Synthesis and characterization at high temperature of colloidal processing cemented carbide and cermet. Tesis (Doctoral), E.T.S.I. Caminos, Canales y Puertos (UPM). 
• Caminos subterráneos, progreso en la superficie. ACCIONA. 
• El túnel de Guadarrama, pionero en la alta velocidad en España (2025). Ministerio de Transportes y Movilidad Sostenible. 
• Delia y Micaela se llamarán las tuneladoras que se usarán para la construcción del túnel de la Línea 2 . Ministerio de Transportes y Comunicaciones de Perú. (2021) 
• ACCIONA inicia la excavación del túnel occidental de la Bahía de Sídney. 2023. ACCIONA.