A few weeks ago, my wife was out walking the dog, and she sent me four or five photos of small orange boxes planted all around our neighborhood. (OK, I’ll bite!) They had little cards on them explaining that they were geophones, and a QR code on them that lead to a website with all the details. Munich was getting a large-scale seismic survey to map out our underground water, with the aim of using it for geothermal heat and power in the near future.

How do you map up to five kilometers under the earth? You pound the ground, sending shockwaves downward, and then listen for their reflections. At the boundaries between different layers, the change in the speed of sound in the different media cause reflections. Calculating the time it took for a given reflection to reach you lets you figure out how deep the layer boundary is.

The seismic survey procedure goes like this: geophones are set out at roughly 20 m intervals in lines spaced around 300 m apart that run roughly north-south, while “vibrotrucks” drive a roughly east-west course, creating mini-earthquakes every 20 meters along the way. Covering a surface of 1,000 km^2 with over 120,000 sample locations and exciting them 86,000 times is going to take a while. Lucky for me, they started in my part of town.

Hot Water

Because the earth is essentially a ball of hot molten metal with a thin and crispy outer crust, and because there is radioactive decay going on even within the crusty bit, temperatures rise as you dig down: roughly 30 °C per kilometer. And Munich has this fantastic source of water that’s folded under the earth in a layer that dates back to the Jurassic, which makes it both low in dissolved minerals and sitting just around 3 km down: at 100 °C. This turns out to be the sweet spot in terms of difficulty of drilling and heat gained from doing so.

Blessed with hot water underground, the question is how to best use it. The plan at the moment is to situate a number of geothermal plants around the city, drill relatively large bore holes at each site that reach down 1 km to 2 km, and then drill diagonally down after that to spider out into a larger source of water. This “extended reach drilling” pulls from a larger heat source, prevents cool spots, and has only recently become technologically feasible.

Which brings us to the GIGA-M study. A 3D map of the underground will help plan out where to put the geothermal plants, in which directions the runners will need to be drilled, and generally how to best coordinate the resource. There were a number of individual smaller surveys done over the last 20 years, and Munich and the surroundings already have a number of geothermal plants, but this survey aims to fill in all of the gaps.

The Hunter and His Prey

It was a Thursday morning when my wife thought she heard some pounding and humming outside. She was wrong, but it got us to look at the online map, and while they weren’t in our neighborhood just yet, they were probably within easy driving distance. I threw my camera and tripod in the car and headed out.

Out in the country, just outside of Grossdingharting (you can’t make these names up!) I spotted a plume of dust rising up from a field. Was it just a farmer tilling? Or was it a vibrotruck? I drove through the woods on a logging road, and when I came out, I was nearly face-to-face with the beasts.

They were loud, and I felt the rumble when I stepped out of the car, so I ran up and asked if I could film it. They said “sure”, and I set up the tripod. The first video I shot, apparently I hadn’t screwed the camera down hard enough in the tripod, and it vibrated loose. So I ran another 20 m further and tightened everything down.

I’m not going to lie: it was exceedingly loud and very exciting. I drove back home and couldn’t wait to review the footage.

The next morning, I heard the same sound outside my own house. They were driving vibrotrucks around in the field about a block up the street. I grabbed the camera and ran out barefoot to get footage. And then Saturday morning, while cooking blueberry pancakes for the family, they drove up my street. Am I stalking the vibrotrucks, or are they stalking me?

An engineer asked if they could stand in our front yard, so I took my chance to bombard him with questions. He seemed stoked to talk about it.

Geophones and Vibrotrucks

When you stand next to one of these things, first it shakes, and then you can hear a bassy tone, and then it rises and stops. They’re obviously doing a frequency sweep, much like you would use a sonar chirp to help disentangle the multiple reflections from one another.

The sweep means that even if they receive two reflections at once, they will hear two different pitches because one signal traveled further than the other, and comes from the earlier, lower-pitch part of the impulse. It also makes for a very easy to find signature. Correlating these times of flight across the entire array of geophones gives a 3D map of the underground layer discontinuities.

You can see from a cleaned up version of the audio that they’re sweeping from something like 6 Hz up to maybe 96 Hz.  The lower end sounds like distinct hammering, and the top end a humming. Here is another video, with filtered audio. Because you feel the vibrations through your feet and in your chest, the live experience is a little bit like this with more noise. Play on good headphones or with a subwoofer for maximum effect.

Of course, the time of flight matters. I asked if each geohpone had a GPS timebase, and the friendly engineer told me that they individual geophones don’t – too expensive – but that when they install one, they connect it to a laptop with GPS that records the location and sets the geophone’s clock. When they harvest them, they record the location again, dump the time to verify that the clocks haven’t drifted too much, and then pull down all of the recorded timestamped data.

Why do they drive around in pairs? It’s simply because they make twice the vibration. The two trucks are actually synced together in phase, so they emit as one. The trucks have a GPS-disciplined clock inside, so they know exactly when they start each cycle and can derive the time-of-flight to all of the receiving geophones. The rest is math.

He also mentioned that it was too bad that I only got to experience the smaller “urban” vibrotrucks like the one outside my house. I kept my mouth shut – nobody needs to know that I’m a vibrotruck stalker.

Fervo's deal with Google could expand to 1 gigawatt of geothermal power, enough to supply a very large AI data center in Utah.

Next month, Endurance Energy will send Adélie — a 100-kilowatt geothermal generator named for a penguin — to an undersea volcano off the Oregon coast, tapping hydrothermal fluids that can reach 750 degrees Fahrenheit.

It’s the Seattle startup’s fifth deployment onto the ocean floor, part of founder and CEO Andrew Redd‘s bet that the technology can one day electrify entire cities.

“Addressing the clean power problem is a way to make significant, scalable impact,” Redd said. “It’s a fundamental of human life — people need power.”

Endurance is part of a broader race to harness geothermal energy — heat from water or steam under the Earth’s crust, converted into electricity. But while companies such as Fervo and Sage Geosystems focus on terrestrial sites, Endurance is the first commercial venture pursuing undersea sources.

Redd left an engineering role at SpaceX to launch his company in January 2025. Endurance’s headquarters sit on the north shore of Seattle’s Lake Union, in a building that once housed seafood processing, just down from Ivar’s Salmon House.

“There was a big crab on the wall. There were pinup pictures from the fishermen that would sleep out back,” recalled Nate Rodland, Endurance’s chief operating officer.

This summer, the team expanded across the street into a 25,000-square-foot concrete facility — it’s cavernous, like a parking garage with ultrahigh ceilings — where giant white tents now house separate R&D and manufacturing operations.

The space is bustling with employees assembling components ahead of the Sept. 7 pilot demonstration.

“Everyone is incredibly multidisciplinary. The challenge is incredibly multidisciplinary. That’s what makes it so fun and exciting,” said Redd, watching his team at work. He pointed to the company’s thermal fluids analyst, for example, who at that moment was assembling electronics enclosures.

The team’s roughly three-dozen members include 12 SpaceX alumni, plus veterans of fusion startup Helion Energy and space venture Blue Origin.

Here’s how the Endurance system works:

• Drill a borehole: The team will drill a narrow hole a safe distance from active vent fields that will go about 200 feet into the seafloor to tap hydrothermal fluid.
• Harness the heat: Hydrothermal fluid flows into a generator, heating a closed-loop refrigerant through titanium tubes until it boils and spins a turbine.
• Reset: Naturally cold seawater cools the refrigerant back into a liquid state to repeat the cycle. The warmed water is released to the seafloor.
• Move energy: The deployment next month will send performance data back to shore via Ocean Observatories Initiative’s Regional Cabled Array, operated by the University of Washington. Permanent installations will move electricity through undersea cables.
• Control the process: Underwater, remotely controlled electronics are housed in oil-filled, pressure-balanced chambers to handle deep-ocean conditions.

“We kind of joke that everything we do is just heating, cooling and pumping,” said Aditi Bhatt, chief of staff, as she pointed out earlier prototype devices housed in Endurance’s original building.

Each unit is named for a sea creature, moving up the food chain as the generators grow — krill, silverfish, and eventually an orca pod. Adélie’s generating capacity would be enough to power a small neighborhood of homes.

The company touts the environmental sustainability of its system, which has a small physical footprint, creates limited noise, releases no toxic chemicals and emits no carbon.

Endurance has done four prototype deployments, reaching a depth of about 2 miles and temperatures up to 726 degrees Fahrenheit. The company has previously visited the Pacific Northwest’s Juan de Fuca Ridge, the Mariana Trough, the East Pacific Rise off Mexico, and the Kingdom of Tonga in the South Pacific Ocean.

While the startup has made rapid progress, the strategy faces real challenges: high pressure and corrosive conditions, mineral scaling inside devices, and the risk of costly repairs to seafloor machinery. And though many population centers sit near shorelines, the systems will still need expensive subsea cables to carry power over potentially long distances.

But the potential to tap a limitless source of around-the-clock, carbon-free electricity is attractive, and the company has raised $63 million from investors.

The next device, at megawatt scale, will power Endurance’s first commercial pilot, with the Kingdom of Tonga. Work on that begins as soon as Adélie is deployed, with the goal of shipping next year.

The pace aligns with Redd’s experience at SpaceX.

“I remember many times working on the next rocket before the previous one had actually launched, just because of how much you learned from the process of building something,” he said, adding that the team is inspired by their mission.

“The sooner we can get clean, low-cost power on the grid, the more people we’re able to help,” Redd said. “So we’re really driving with intensity.”

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