A wall of white granite towered above a small team of engineers gathered in a California quarry. The ancient, ultra-hard material before them was, to most things, completely impermeable.
Then the rock-melting tunnel-boring machine (TBR) roared to life.
"It's kind of like igniting a rocket," says Troy Helming, founder and chief executive of EarthGrid, as he describes the initially loud process of lighting up three plasma torches at the front of his company's machine.
Those torches, set within a spinning head, soon quieten down as they produce a stream of super-heated plasma reaching 27,000C - significantly hotter than the surface of the Sun.
During the California test this January, EarthGrid's cigar-shaped boring machine chewed through three metres of granite. "We create, basically, a tornado – a violent vortex inside the tunnel," says Helming, as he explains how this helps the machine to suck away debris, which at times takes the form of lava.
"I actually got a little bit emotional watching it," adds the entrepreneur. "I've been waiting for this moment for 10 years."
Emerging technologies like this could make tunnel boring quicker and easier. Putting electricity or telecommunications cables, substations, data centres and other critical infrastructure underground, while good for securing such equipment, has long been a very expensive and difficult option.
Engineering firms told BBC News they are seeing rising demand for undergrounding, in part due to Russia's war with Ukraine, which has revealed just how vulnerable above-ground facilities can be to drone attacks.
Helming says he has fielded interest from companies that want to use his tunnel boring machine for power and fibre optic cables, or pipelines that could transport water, natural gas, or ammonia, for example.
One project the company has eyed up would involve boring tunnels for an underground freight-distribution system around airports and warehouses. "To take more trucks off the road," says Helming.
The January TBR test went well, though the machine "over-bored" slightly to the top and left of the tunnel, says Helming. His team plans to adjust the machine so that it will create a vortex that spins in alternate directions every five minutes or so, in order to correct this, and they hope the TBR could see commercial deployment as early as next year.
Humans have long buried things in the ground to protect them. Doing it safely and cost-effectively with modern infrastructure is hard but sometimes, even in remotest places, it's an option worth taking.
"What we're seeing is that brutal materiality is still important," says Alexander RE Taylor, senior lecturer in communications at the University of Exeter. He has identified what he calls a "data bunker boom", external in which data centres are increasingly going underground.
One data centre completed earlier this year was installed in a corner of the Dolomite Mountains, external in Italy.
Freshly excavated caverns 100m underground sit next to stores that have been used for sparkling wine, apples and cheese in recent years.
Trentino DataMine's naturally cool space makes it cheaper and less energy-intensive to keep servers cool.
Chief executive Dennis Bonn adds: "Ninety million cubic metres of dolomite rock provide natural protection against physical intrusion, electromagnetic interference, seismic events and hydrogeological risks - levels of protection that simply cannot be replicated above ground."
Elsewhere, some tech is burrowing into the seabed. Subsea internet cables, which may run for thousands of kilometres beneath oceans, are occasionally damaged by ship anchors.
There is evidence to suggest, external that, as burying part or all of the cables has become more common, faults with these cables have become rarer, per kilometre of deployed cable, explains Lane Burdette, senior analyst at TeleGeography, a telecoms market research firm.
"Submarine cables are increasingly being buried up to three meters deep," she says. "In some fault-prone areas, they are buried along their entire lengths."
Taylor says various examples demonstrate that keeping things underground, or deep inside caves, is an effective defensive strategy.
"The tunnels that Al-Qaeda were using were a major strategic problem for the US [during the war in Afghanistan]," he adds. More recently, Iran's notoriously difficult-to-bust underground nuclear facilities have gained much attention.
The war in Ukraine, and other geopolitical events, have "definitely had an impact" on demand for undergrounding in some areas, says Robbie McGoran, head of work winning and business development at Joseph Gallagher, a civil engineering and tunnelling firm.
Countries bordering Russia are increasingly asking about undergrounding. "They're very cautious about who they'll even let do their work – and also burying [infrastructure] and making sure it's well protected. We are noticing that," he adds.
McGoran also says that technologies such as laser-guidance systems and gyroscopes, which help machines work out their position relative to the world around them, have made tunnelling more accurate in recent years.
However, there are still significant challenges, from releasing potentially dangerous gases locked in the ground to occasional flooding issues. Dramatic innovation in tunnel boring equipment, significantly increasing the rate at which tunnels can be carved out, does not come along very often.
"We usually measure [progress] in millimetres per minute," says McGoran.
In the UK, there isn't a noticeable shift towards putting critical infrastructure underground yet, says Mark Neller, energy leader for Europe, India, Middle East and Africa at engineering consultants Arup.
Though in some places, tunnels are clearly necessary. Neller and colleagues worked on the £1bn London Power Tunnels project, which involved building 18 miles (29km) of tunnels under London to house large electricity cables.
This was the perfect choice for such a busy urban area but because tunnelling can be several times more expensive than above-ground cable infrastructure, Neller says that, elsewhere, simply installing additional circuits can provide sufficient resilience.
"That's actually a much more cost-effective way," he says. "The electricity system [in Great Britain] is designed with quite a lot of redundancy built into it."
Richard Little, infrastructure policy consultant and editor of the Journal of Critical Infrastructure Policy, remembers the latter days of the Cold War, when it was the threat of all-out nuclear war that made bunker-building a frequent talking point in the West.
To this day, Switzerland mandates that every citizen must have access to a nuclear bunker, external. Many apartment buildings in the country have such facilities in their basements.
"A lot of it was real Dr Strangelove stuff," says Little, referring to discussions around bunker-building during President Ronald Reagan's administration. "The world's going to end but I guess we'll maybe harbour a few hundred critical people down here and they'll survive."
Little, who authored policy documents regarding underground critical infrastructure, external during the 1990s, adds, "It became obvious rather quickly that you can't put everything underground."
Today, it might be worth identifying specific facilities that would be difficult to restart or replace, were they attacked, as candidates for undergrounding. "The first thing that occurred to me was [computer] chip manufacturing," says Little, offering an example.
"Underground facilities, I'm sure in certain instances, would make a great deal of sense – but it's all about what's critical."
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