Run Silent, Run Deep: The Case for a Subsea Hyperloop

Run Silent, Run Deep: The Case for a Subsea Hyperloop

Blake Cole
Marine Engineer, Hyperloop One

Think of it.
On the surface there is hunger and fear. 
Men still exercise unjust laws. 
They fight, tear one another to pieces. 
A mere few feet beneath the waves their reign ceases, their evil drowns. 
Here on the ocean floor is the only independence. 
Here I am free! 

-Captain Nemo, Twenty Thousand Leagues Under The Sea


For most of the past century, the tunneling industry has been grounded in, well, the ground. Most of us have seen photos of giant boring machines gouging throughways out of the rock, sand, and gravel dozens of yards beneath the surface. The prospect of building tunnels through water was often viewed as fringe. It was seen as needless, expensive, dangerous, and complex. But in the last decade or so, advances in engineering, materials, and design have allowed us to build underwater tunnels at an unprecedented rate, at increasing depths, far faster, and at a lower cost than ever before. The most prestigious engineering firms in the world are allocating more time and resources into underwater tunneling, a move likely to kick in further gains in cost, safety, and reduced complexity. 

Hyperloop One has had subsea aspirations almost since our inception in November 2014. We’ve alluded to it in press releases, and hinted at routes that would take passengers or cargo under the sea at the speed of sound. It’s no secret any more. Our partners at FS Links recently unveiled a 500-kilometer Nordic route that gets people from Helsinki to Stockholm in under 30 minutes. Part of that journey includes a 200-kilometer stretch dipping above and below the Åland archipelago in the Baltic Sea between Finland and Sweden. This international stretch would incorporate one of the world’s longest water tunnels ever built, estimated to cost $6 billion out of the $21 billion total capital budget for the project. As the company’s first and, for now, only marine engineer, one of my jobs is to help figure out how to safely and affordably construct a Hyperloop under water.  

The big question for me is: Do we tunnel under the sea floor (traditional approach) or do we go through the water (not so usual)? For a rider, the trip would feel no different than one over (or through) land; the transition from land to water will be so smooth as to be imperceptible, and the duration of the subsea segment will be so brief, at least initially, that the entire experience will last only seconds or minutes. The rider will happily emerge from his or her pod at the end of their journey, having traveled faster beneath the waves than any living creature in Earth’s history. 

In many ways, designing the Hyperloop to operate in an underwater environment is no more complex than adapting the system to function within bored-rock tunnels. Most of the pod levitation and propulsion systems will be directly transferrable, and a great deal of the vacuum technology will be adaptable to the subsea environment, with some relatively minor alterations. Both systems require intricately planned ventilation systems and egress protocols; both systems must contend with extremely high-pressure external environments; both systems must preclude water leakage and collapse, at all costs.  

The big challenge to underwater tunneling is the industry’s collective lack of experience. Many tens of thousands of earthen tunnels have been constructed worldwide. Engineers are comfortable with established bored-tunnel construction techniques, but these techniques were hard learned over decades and even centuries. Underwater tunneling has yet to experience the same coming of age. 

Subsea tunnels fall into three distinct categories. The oldest, and most conventional variety are called subsea bored rock tunnels, a methodology more or less identical to terrestrial bored-rock tunneling—although highly-saturated soils present a few additional challenges. A notable application of this technique was the construction of the Channel Tunnel from the United Kingdom to France. Its construction began in 1988 and it opened for traffic in 1994. 

A section of an immersed tunnel before its installation under the Bosphorus Strait in Turkey. Credit: Wikipedia.

The second variety of underwater tunnel, and by far the most prevalent as of late, is called an immersed tunnel. Only around 200 immersed tunnels have been completed in past hundred years, according to engineering firm Ramboll. This type of tube employs precast steel or concrete segments resting on the benthos, or sea floor. In most shallow waters, immersed tunnels are cheaper and faster to construct than bored rock tunnels. And, because submerged tunnels are going into shallower waters they typically require shorter transitional tunnels from the land mass on either side. Famous examples include the TransBay Tube in San Francisco and the Oresund Bridge Tunnel between Denmark and Sweden. Here’s a great animation showing how the giant Fehmarnbelt immersed tunnel will be assembled and installed between Denmark and Germany.  

The complexity and cost of immersed tunnels depends largely on two items: tunnel length and bathymetry. Bathymetry is an oceanographic term meaning “underwater topography.” If the seafloor is smooth, the maximum water depth is less than, say, 100 meters, and the required tunnel length is on the order of kilometers to tens of kilometers, an immersed tunnel will probably suffice. The deal-breaker with immersed tunnels is depth. For every 10 meters' increase in depth you increase the amount of hydrostatic pressure on a submerged tube by the weight of one full atmosphere (~12 km of air). At a certain point no hollow structure can hold up to this staggering compressive force, which is why no immersed tunnels have been built at depths below 70 meters. Immersed tunnel ‘best-practice’ is constantly evolving, and great leaps in cost-reduction and efficiency are made with each subsequent innovation. We plan to be part of that evolution. 

The only solution we’ve seen for crossing waters deeper than 70 meters and over distances of 100 kilometers or more is the submerged floating tunnel (SFT). Also called an Archimedes’ Bridge, this tunnel employs the buoyancy of the tube itself, in conjunction with stabilizing tension cables, to traverse the underwater environment at a fixed distance below the surface. Some anchorless systems have been considered, though not in great detail. Because the SFT hovers off the bottom, the system is largely unaffected by undulations and obstacles on the sea floor. And because it is anchored at least 20 meters down, it avoids the highly turbulent surface layer of the sea. Of course, the advantages are met with many engineering challenges. An SFT, unlike an immersed tunnel, still has to deal with waves and currents, changes in water density and local variations in buoyancy, not to mention the possibility of a collision with ships, macrofauna, and submarines. Corrosion is also a big issue. 

A rendering of an automotive submerged floating tunnel. The photo at top shows a rendering of an SFT from the surface. Credit: Vegvesen

.Add all these challenges up and it’s no wonder that no SFT has ever been constructed. But academic studies on the topic have blossomed in the past decade. Since 2010, Statens Vegvesen, the Norwegian Roadway Authority, has invested extensively in various SFT prefeasibility studies. Given its multitude of narrow fjords too wide to bridge and too deep to tunnel underneath (some bottom out at 1,300 meters below sea level), it is no surprise that Norway is particularly enticed by the SFT concept. South Korea and China have also expressed interest in the idea as of late. 

We can most easily deliver a subsea Hyperloop via a bored-rock tunnel (the approach we will likely take across the Baltic Sea) or an immersed tunnel in the right environmental conditions. But that would be too easy! Successfully building an underwater Hyperloop using a submerged floating tunnel will score us double entries in the history books: a novel form of transportation through a novel conduit of traffic.  

Blake Cole graduated from UC San Diego in 2013 with a BS in Environmental Engineering and got his MS in Environmental Fluid Mechanics from Stanford University in 2015. He's a surfer and scuba diver and has worked at Hyperloop One since 2015. You can reach him at