Inside the world’s deepest and longest subsea road tunnel

Welcome to MIT Technology Review Narrated. My name is Matt Honan. I'm our editor-in-chief. Every week, we'll bring you a fascinating, new, in-depth story from the leading edge of science and technology, covering topics like AI, biotech, climate, energy, robotics, and more. Here's this week's story. I hope you enjoy it. Narrated by NOAH. Listen to more of the best articles from the world's biggest publishers on the NOAH app. Or at newsoveraudio.com. Niall Firth writes, Inside the World's Deepest and Longest Subsea Road Tunnel. It's cold, it's very, very noisy, and if I can be quite honest with you, I'm not feeling super relaxed. I'm currently around 300 meters or 1,000 feet beneath the North Sea in a dark, dank cave. It smells weird, and I am increasingly aware of the pressure from millions of tons of seawater just above my head, pushing down with a force of more than 500 pounds per square inch. Picture a baby rhino standing on a postage stamp. Only fabulous engineering is keeping me from being crushed, drowned, disappeared. My safety goggles are foggy. Just a few hundred meters away, someone is about to blow up a giant rock wall. Luckily, earlier that day I was given a full safety briefing, and I've got a special hard hat on. "Don't worry if you don't make it, we'll have your stuff sent back to your office," geologist Anne-Maretta Gjølje tells me, straight-faced. Ah, Norwegian humor. I'm in this odd situation under the iconic fjords of Norway to visit what will soon become the world's longest and deepest subsea road tunnel. Called Rogfast, short for Rogaland. I want to understand how you make something as audacious as a 26.7-kilometer or 16.6-mile highway that sits 390 meters or 1,280 feet below the sea at its deepest point, and also at a time when it can feel hard to get anything done, especially in the US, to reassure myself that ambitious engineering is still possible. That we can still make things. The Norwegians already have the world's longest subsea tunnel. The 14.4-kilometer Refylke through Rogfast will dwarf it. Their expertise has attracted attention from Japan, Spain, Morocco, and even a number of U.S. states, whose representatives were due to visit the site in May, just weeks after I went. They too want to know how Norway does it. The answer? Tons of explosives. The entire endeavor feels like an obstinate refusal to give in to physics and geology. "It's always exciting," Nicolas Bruched, a tunnel foreman at Implenia, a Swiss firm involved in the project, tells me. "Every blast creates a new world. There's not just the blasting of the tunnel itself, although that is an epic project on its own, but an immense logistics challenge involving huge ventilation shafts, Extreme pressure, underground roundabouts in the complex Norwegian geology. Oh, and the water. So much water. This is the longest continuous blast on the sea, says John Olaf Osterhaus, assistant project manager at Implenia. Never been done before. We can't buy a book to see how we do this. Alright, time to fish my phone out of my safety suit. Don't want to forget this. Arriving at the rock face where the tunnel hits seabed feels like being on the moon. It's a huge slab of stone at the end of a long, dark, wet, wide passageway that's lit, barely, by electric lights. Giant vehicles carting tons of rocks rumble past periodically, and we pull to the side of the road to let them by. Workers clock in for 12-hour shifts, 6 AM until 6 PM, deep in the bowels of the earth, where no natural light can reach. 12 days on, 16 days off. They eat their lunch at a table in this damp cave surrounded by porta-cabins plastered with safety notices. It's kind of a lifestyle, says Bruset, laughing. You have to be a little bit crazy to work underground all the time. These crazy engineers are here to make tunnels the Norwegian way. The nation frequently uses what's known as the drill and blast method instead of the tunnel boring machines that are more typical elsewhere. This approach offers more flexibility for long, complex operations with varied rock types. Each blast adds about 5 to 6 meters to the tunnel. Rogfast is being built inward from the ends to speed things up. The construction company Skanska is leading from the north, coming from the island of Vesterborken. Implenia has joined a company called Stangland to tunnel from Randeburg in the south, which is where I am. Both teams use multiple laser scans each day to consistently measure their orientation and check that the tunnel is exactly where it should be. The two ends should meet sometime in 2029 with no more than just a few centimeters of deviation. Norway has constructed more than 1,000 kilometers of tunnels over the past several decades, The depth and length of these make the best efforts to date of Elon Musk's Boring Company, a mere 2.7-kilometer tunnel in Las Vegas that is just 3.6 meters wide, look rather pathetic. The country's spectacular setting makes such builds necessary. While Norwegians are proud of having the second-longest coastline in the world after Canada, getting up and down the west coast requires multiple ferry rides between islands. Which can move extra slowly when the weather's bad. After it's completed, which is scheduled to happen in 2033, Rogfast should help eliminate two ferry routes and cut the 5-hour journey between the southwestern cities of Stavanger and Bergen by 40 minutes. It will funnel 4 lanes of traffic deep beneath the fjords of Boknefjord and Vitsøyfjord, and at one section, a relatively scant 50 meters of rock will separate the drivers speeding through the tunnel from the bottom of the North Sea. There are also delightfully two undersea roundabouts located 220 meters below sea level. But the first job is to contend with all that water. Subsea tunneling is defined by a constant, ultimately unwinnable battle with the ocean. The sheer weight of the sea above you and the crushing pressure means the water will always find a way in. "It's the volume and the pressure that's the biggest risk," says Ali Magna-Renning, project leader for Implenia Stangland. So before tunnel engineers blow stuff up, they need to check for leaks. Into the rock face ahead of them, they drill a number of narrow holes that go 25 to 30 meters deep to see how much water comes through. Even a small probe can unleash a torrent within seconds, says Renning. When road traffic eventually rumbles through these tubes, water will still trickle from the rocks. It will be redirected into many reservoirs dotted throughout the tunnel network before being pumped back out. Since stopping the water entirely is impossible, the game is instead to push it away as best you can. If the leakage in front of the rock face exceeds a certain limit, around 4 liters per hole per minute, then the next stage is grouting, pumping a mixture of cement-like sludge into new holes that fan out in the ceiling above and around the face. Ideally, you address the leaks that are ahead of you, "It's a lot more difficult to stop a leak that's behind you," says Renning. At one point deep below the sea, I chat with Torald Johan Nomland, the project's grouting specialist. He's big and bearded, perhaps one of the most Norwegian-looking men I've ever met. He stands towering above me and shakes my hand in his giant bear-like paw. Grouting is in Nomland's family. His dad did it too. He loves it. There's not necessarily just one solution to a problem, he says, eyes flashing with delight as he describes fighting the interminable battle with the water. There may be many solutions. The amount of grouting needed determines how fast the project can move. On the Skanska side, for example, some weeks the face moves 30 meters, others as few as 10. This isn't made any easier by the rock itself. The seabed around Norway was shaped by glaciers during the Ice Age. As the ice retreated, it dragged softer rock with it, carving out the fjords for which the nation is so famous. But this legacy makes digging subsea tunnels particularly gnarly. Much of what's left is the hard, difficult-to-break stuff. And it's not just one type of rock either. There are big, wide areas where we don't know what's down there. Says Guilherme, the geologist who is a project manager for the Norwegian Public Roads Administration, which is in charge of the entire project. Before any construction started, boats took core samples from the seabed along the planned tunnel route. Seismic surveys from the ocean surface, like those that look for oil in the region, helped fill in the gaps. Each kind of rock presents its own challenges, so the engineers have different techniques for different problems, Guilherme explains. For example, they found that one southern section contains a lot of phyllite. Phyllite is considered nice to work with. It is formed from a combination of shale, siltstone, and mud over time and is pretty compact with few cracks to let water through. Its compact nature means it requires more explosives per blast, however. It also contains a lot of quartz, which is toxic when released into the air during blasting. So workers wear monitors to measure their exposure, and a curtain of water sprayed in front of the rock face helps prevent too much from drifting into the tunnel. The northernmost part of the route, meanwhile, is made mostly of solid granite and a similar rock called gneiss. Both are hard but contain fractures that allow the seawater to trickle through. The rock type can also change over just a short distance. So during the dig, every 80 meters or so, an engineer sends sound waves through the face to expose its secrets and help evaluate its structural integrity. The rock is graded on a scale of 1 to 5, with 5 being the worst and least stable. When you are reaching Class 5, then it's almost like soil. It's not rock anymore, says Renning. This investigation informs the kinds of structural supports each section will need, from steel rods that fan out above the rock face like an umbrella for the strongest rock, to reinforced concrete arches that hold up the weakest. To seal everything off, the team sprays a substance called Shotcrete—liquid concrete mixed with reinforced steel fibers—onto the walls throughout. A plastic membrane and concrete panels are fitted later. It's going to be a very safe tunnel, Gillier says. It's going to last for 100 years. While I may not be brave, at least I don't get seasick. Back at the surface, I board a small ferry that putters and sloshes its way from the mainland to Vitsøy, a sparsely populated municipality made up of 365 separate islands and islets, something its 550 or so inhabitants are very proud of, even though most of these islands are uninhabited chunks of rock. For the next few years, Vitsøy's population will experience a tiny boom, as its largest island hosts a semi-permanent encampment of contractors and engineers working on what is probably the most complex part of the Rogfast project: the giant ventilation shafts that will sit roughly halfway along the tunnel's length to bring fresh air into the entire network and remove the stale air in turn. It's also one of the reasons why road tunnels are much more complex than rail tunnels. Cars pump out fumes that have to be vented away. During construction, fresh air flows in via huge plastic tubes suspended from the ceiling, but eventually Rogfast's air will come in through two 9-meter-wide shafts that will bore down from Vitsøy's surface—one to bring it in, one to take it out. Creating these shafts is a wild process. First, narrow boreholes are drilled from the ground down into the tunnel 210 meters below the surface. A vertical drill rig is then pulled up through the hole from the bottom, widening the shaft to 2.4 meters as it ascends. Then explosives are set off on the island's surface, bashing down through the rock to widen the shaft. A large digger pushes the resulting debris down the narrower, not-yet-exploded length of shaft below, sending rocks barreling toward the tunnel at the bottom, like socks tumbling down a laundry chute. Trucks haul away the fallen rocks. This process happens in stages, repeating at regular intervals, opening up the passage a bit deeper with each pass. Once it's all done, steel rods are installed in the shaft's walls to keep it secure. Down below, I stand beneath one of the narrow guide holes for one of the two ventilation shafts. The ceiling soars overhead, a strangely beautiful cathedral, cragged and shadowed by lamplight. Besides poisonous air, the epic nature of these engineering projects throws up other surprising dangers. For example, Rogfast will take about 30 minutes to drive through. It doesn't seem that long, but the project's designers worry that the monotonous environment may lull some drivers to sleep. Engineers faced this problem with Rifilkær, which as the current longest subsea road tunnel has been a testing ground for its bigger sibling. It relieves the tedium with a large hall that opens up in the middle of the tunnel, lit by coloured lights that change each day. When Rogfast is finished, Artists will be invited to do something similar, using lights, colors, and shapes to keep drivers alert. Then there are the environmental risks. What is there to do with all the loose rock created by the blasts? The engineers predict 8.5 million cubic meters worth. That's enough to fill more than 2,500 Olympic-sized swimming pools. The solution is to bring it back to the surface where it can be used to create new land. To do this, the project employs a giant barge designed to split open and dump 350 tons of rock in one go. But adding more rock particles to the water can make it hard for fish to breathe, says Elisabeth Ørstorpalen, Implenia's environment lead on the project and my fellow passenger on the windy and soon-to-be redundant ferry over Vitse. Her team monitors their levels in real time. If the particulate count is too high, the drops must pause until the new rock has settled on the seabed. The goal is to protect lobster fishing, a vital part of the local economy, and to safeguard the breeding time for cod, which was an issue when I visited. Finally, of course, on top of all this are the many hazards for the people who are actually doing all this blasting and digging and hauling. Or, say, for the visitors who are finding their inner 9-year-old getting a little too giddy about what's next. Before I'm allowed underground, I must sit through a short safety briefing where I learn there are multiple hazards when you're that deep. Fires, for instance, can break out, exacerbated by the way the salt water affects electronics. Just a week earlier, a car caught fire somewhere deep within the network. You have to be aware all the time, says Anne Britt Møwen, the project lead for Skanska. It's a very harsh, harsh climate. After the session, I'm given a high-vis suit, the hard hat which has built-in ear protectors, gloves, safety glasses, and reinforced boots. I get instructions on how to operate the oxygen mask that will be in the car with me, and a device to put in my pocket that will track my exact location on screens in the control room. The device also acts as a personal warning system. If it vibrates and a blue light appears, then a blast is imminent and I must get to safety. If it vibrates and glows red, um, well, that's bad news and it's time to evacuate. But let's say I can't, I'm too deep underground. Then there is a second, less fun option. I'm given instructions on how to access the rescue chambers. These metal boxes, about the size of a large van, can squeeze in around 16 people, and each contains chocolate, water, radio equipment, a defibrillator, and enough oxygen for 24 hours. I see them dotted throughout the tunnels as we drive through. Worst case scenario, I'm supposed to get to the nearest one, sit tight, and hope to get rescued. If you're the first to the rescue chamber, press the green button for 15 seconds to release pressure. Says Kjetil Myklebost, a project manager at Implenia. "And then close the hatch and sit down and be calm." Calm. Right. Okay. In the hours before my visit, a huge drilling jumbo rig puts as many as 180 holes deep into the rock face. The number, angle, depth, and spacing of the holes is calculated in advance using software but finalized at the face. Here they're almost 6 meters deep, At one point I clamber up into the jumbo and inspect the pattern on its screen, matching it against what I can see on the huge rock face which stands more than 12 meters tall and wide. The holes have been stuffed with an explosive slurry. Someone quips that if I get any on my clothes I'll be stopped at the airport as a terrorist—a Norwegian joke, again. As I watch workers in a kind of cherry picker fit each hole with the detonator, and make sure they're all connected to one another by wire, ready to be triggered remotely. Then my personal safety device starts vibrating. When I take it out of my pocket, it's blinking blue. Showtime. "How far back do I need to be?" "It's dangerous in this direction, 500 to 600 meters, but if you're around the corner, you can be closer," says Sveinug Brod, project manager for the Norwegian Public Roads Administration. I stand by the worker who will trigger the blast from what looks like a small briefcase with an antenna. Then he presses the button. The shockwave hits me before I hear it. My chest vibrates. In the first few milliseconds, a propulsive thump briefly stuns my senses, followed immediately by a rolling, crumpled thunder. Just a moment later, almost instantly really, wind billows through the cavern. Rocks clatter as they crash off the walls. I try not to show any panic. That was meant to sound like that, right? A hush falls, and there is just the tinkling of stones as they bounce and skip amid the rubble. Dust rises into the air, and there is a strange smell. Through my ear protection, it sounds like the end of the world. The explosion itself is a beautiful choreography. Blasts are initiated one after another, starting from the center. In video footage you can just about hear the sequential pitter-patter of the charges as they go off. In person it's a bit more all at once and overwhelming. Rogfast has just crept another few meters closer to completion. I find myself grinning. Maybe there's something extremely primal about being near an explosion? I'm not sure. I look down at my hand where I have my phone out recording the intensity of the moment except I wasn't recording. The stupid rubber safety gloves I'm wearing must have stopped the command from going through. Oh no, oh no! A once-in-a-lifetime opportunity, and I, uh, blew it. I wasn't recording! I shriek. It's better that way, says Renning, walking off into the gloom. You'll remember it. How very Norwegian. You were listening to MIT Technology Review, where Nyle Firth writes: Inside the World's Deepest and Longest Subsea Road Tunnel. This article was published on June 22, 2026, and was read by Sam Scholl for NOAH.