What if tunnels could be dug anywhere?

Trains zoom through tunnels under hills and rivers.

What if we could dig a tunnel anywhere we liked?

Imagine you could dig a tunnel anywhere: straight through a mountain, under a wide river, or beneath a busy city. Trips could be shorter and roads could stay out of sight.

But real tunnel builders cannot just dig wherever they want. Some ground is hard rock that must be cut or blasted; some is soft mud that would fall in; and almost everywhere, water seeps toward any hole. In this lab we follow three clues: hard rock and soft mud, tunnel-boring machines, and tunnels under rivers.

Every tunnel is a negotiation with geology. The ground's strength, its fractures, its temperature and the water it holds decide whether a tunnel is possible at a given site, how it is excavated and supported, and what it costs.

This thought experiment imagines a world without those constraints and then asks what engineers actually do about them. It follows three threads: hard rock versus soft ground, the tunnel-boring machine (TBM) that digs and lines in one pass, and the long struggle to tunnel beneath rivers. History and engineering come from Britannica, HS2, the American Society of Civil Engineers and Guinness World Records; the 'dig anywhere' world is a set of ideas.

Some ground is hard rock. Some is soft mud.

And water likes to leak into holes. Tunnel builders must hold back the rock, the mud and the water.

Britannica sorts tunnel jobs by the ground they pass through: soft ground, hard rock, soft rock and ground under water. In hard rock, crews have long drilled holes and used explosives to break the rock. In soft ground, the danger is that the earth caves in before it can be supported.

Engineers talk about stand-up time: how long the ground will stay up by itself before it needs support such as steel ribs or a concrete lining. Water makes everything harder. Britannica notes that most long rock tunnels have had problems with water flowing in.

Britannica classifies tunneling by ground type (soft ground, hard rock, soft rock and subaqueous) and describes common stages: investigation, excavation and muck removal, ground support and environmental control. Engineers assess stand-up time, how long an unsupported opening remains stable, to decide when to install support.

Stable openings rely on the ground-arch effect, which transfers load around the opening to its sides; steel ribs and concrete linings share that load. Weak ground calls for smaller headings supported in stages. Most long rock tunnels have met water inflows, and unexpected ground or water can stop work, change designs and raise costs, which is why geological investigation comes first.

Let's pretend we can dig anywhere, easily.

Maybe trains could go straight through every mountain! But real ground can be hard, soft, wet or hot.

If tunnels could be dug anywhere, easily and cheaply, maybe railways would run straight through mountains instead of winding around them, and roads could go underground beneath crowded cities. Those are our ideas, not predictions.

In the real world, the ground pushes back. When the Simplon railway tunnel was dug under the Alps around 1900, Britannica says, crews met rock that burst from the walls, soft rock that swelled and squeezed in, and water as hot as 130°F (54°C). Deep underground can be a hot, wet, high-pressure place.

Remove geological limits and the obvious ideas follow: straighter, shorter routes through mountains and under cities and water. Whether they would be worth building is a separate question of cost and use.

Real geology imposes those limits. Britannica's account of the Simplon tunnel (1898–1906), nearly 7,000 feet below the mountain crest, lists rock bursts from highly stressed rock, swelling weak schists and gypsum that required 10-foot-thick masonry lining in places, and water at 130°F (54°C). Driving it as two parallel tunnels linked by crosscuts helped ventilation and drainage. Deep tunneling is a fight with stress, heat and water.

Today, giant machines called tunnel boring machines dig tunnels.

They cut the ground at the front. Then they build the tunnel wall right behind them!

Modern tunnels are often dug by tunnel-boring machines, or TBMs. The first successful modern one, the 'Mittry Mole,' dug through soft rock at Oahe Dam in South Dakota in 1954. Since then, Britannica says, machines like it have raised tunneling speeds from 25 to 50 feet a day to several hundred feet a day in good ground.

A TBM is like a factory underground. HS2, a British railway project, describes its machines Florence and Cecilia, each weighing 2,000 tonnes, digging through chalk and lining the tunnel with precast concrete segments, grouting them into place and moving forward about 15 to 16 meters a day.

The modern tunnel-boring machine, or mole, dates from the 'Mittry Mole' of 1954 at Oahe Dam, South Dakota, in readily cut clay-shale. Since 1960, Britannica notes, moles have raised advance rates from 25–50 feet per day to several hundred feet per day.

A TBM excavates and supports in one continuous process. HS2's two 2,000-tonne machines, Florence and Cecilia, finished a 10-mile twin-bore tunnel under the Chiltern Hills in early 2024, at depths up to 80 m, excavating three million cubic meters of chalk. Each worked as an underground factory, installing 56,000 precast concrete segments, grouting them and advancing about 15–16 m per day on average. Greathead's 19th-century shield already erected permanent lining segments under its protective tail.

Long ago, digging under a river was very hard. Water kept getting in!

A clever shield helped workers dig safely under the River Thames in London.

For a long time, tunneling under rivers was thought impossible. Then engineer Marc Brunel invented the tunneling shield, a strong frame that protected workers while they dug. With his son Isambard, he began a tunnel under the River Thames in London in 1825. Water broke in several times, but in 1841 they finished the world's first true underwater tunnel, 1,200 feet long.

Later engineers added compressed air inside the tunnel to hold back the water. Since 1950, many underwater tunnels are made a different way: long tube sections are built, floated out, sunk into a trench and joined together.

Britannica credits Marc Brunel with the protective shield, first used with his son Isambard in 1825 on the Wapping–Rotherhithe tunnel through clay beneath the Thames. After several floodings from sand pockets and a seven-year pause, they completed the world's first true subaqueous tunnel in 1841: nine years' work for 1,200 feet.

In 1869 Peter Barlow and James Henry Greathead built a second, smaller Thames tunnel in one year using a circular shield and cast-iron lining segments; in 1874 Greathead added compressed air inside the tunnel to resist water pressure. Since 1950 most underwater crossings have used immersed tubes, prefabricated sections floated to the site, sunk into a dredged trench and joined, first used in this form for the Detroit River railroad tunnel (1906–10).