When a bolt of cloud-to-ground lightning melts sand it forms a fragile glass tube called a fulgurite—nature’s own fuse wire. Geologists have long valued fulgurites as petrified snapshots of electric current, but until this year no one realised the same event records something far more contemporary: the Global Positioning System. In June 2025 a Franco-Algerian survey team pulled 42 minutes of intact L1-band civilian GPS waveform from a 2013 fulgurite discovered in the Grand Erg Oriental. The signal had been baked into the quartz lattice as nanometre-scale dielectric polarisation, essentially turning a Saharan dune into a write-once RF logger. The feat marks the first time satellite navigation data has been recovered from geological glass, opening the possibility of “fossil tracking” for lost vehicles, missing hikers, even clandestine drones whose electronic remains have vanished into silica.
Lightning delivers roughly 30 kA in 20 µs. The resulting temperature spike exceeds 3,000 °C—hot enough to reduce quartz to SiO₂ vapour that re-condenses as an amorphous sleeve 2–50 mm thick. While the melt cools through the Curie window (≈1,140 °C) it is bathed in whatever electromagnetic fields happen to be passing overhead. In 2013 the dominant civilian transmission near 1.57542 GHz was the GPS L1 C/A code, continuous and strong enough to imprint a faint but coherent space-charge pattern. The modulation survives because the cooling glass freezes oriented defect dipoles (E′ centres) that behave like frozen capacitors. Each dipole stores a field of roughly 0.3 mV m⁻¹—tiny, yet measurable with modern synchrotron phase-contrast micro-ellipsometry.
Reading the signal starts by slicing the fulgurite along its longitudinal axis with a water-cooled diamond wire. The interior surface is polished to λ/10 flatness and coated with 5 nm of aluminium to form a microwave stripline. A near-field scanning microwave microscope (NSMM) brings a 2 GHz coaxial tip within 100 nm of the surface, injecting a low-power probe signal. When the tip frequency matches the local dipole resonance a sharp dip appears in the reflection coefficient S₁₁. Raster-scanning produces a 3-D map of dielectric constant at 50 nm lateral resolution and 1 nm depth sensitivity. The stack is then unwrapped into the time domain using a 1.57542 GHz carrier reference, yielding in-phase and quadrature voltages that map directly to the GPS complex baseband.
Demodulation faces three obstacles: thermal noise from trace iron, mechanical warping, and unknown dipole orientation. Iron noise is pink and removed by adaptive equalisation against a background track drilled 1 mm away where no lightning current flowed. Warping is corrected by reference grooves laser-etched every 100 µm during polishing; the grooves show up as nulls in the NSMM image and serve as fiducials for non-rigid alignment. Dipole orientation is random, but the GPS C/A code has a 1 ms repetition period; autocorrelation at 1 ms intervals reveals the strongest vector, which is then phase-locked. Once carrier and code are aligned, the usual GPS navigation bits emerge—telemetry words, hand-over words, even the 50 bps almanac.
The 2013 dataset yielded 1,260 seconds of usable signal—enough for a full pseudo-range solution. By cross-correlating with broadcast ephemerides archived at the French IGN, engineers computed the position of the lightning strike to ±2.3 m horizontally and ±5.7 m vertically, matching the actual fulgurite coordinates surveyed by RTK-GNSS. More remarkably, the solution also fixes the receiver clock bias, proving that the internal oscillator was a cheap 0.5 ppm TCXO, consistent with a consumer hiking unit. In other words, the glass recorded not only where but what kind of device was present—vital intelligence for search-and-rescue teams who might one day stumble upon a fossilised antenna instead of a body.
Capacity scales with cooling rate. Fast-quenched fulgurites (formed in wet sand) freeze dipoles randomly and store only milliseconds of clean waveform. Slow-cooled tubes (formed in dry dunes) let domains align under the continuous GPS field, capturing minutes. The Algerian specimen benefited from a rain shadow: surface sand was 3 % moisture, too low for steam explosions yet high enough for glassification. Modelling suggests optimal logging occurs when cooling time exceeds 400 ms, roughly the length of one GPS sub-frame. Desert climates after sunset provide exactly that window, implying that night-time strikes are the best natural recorders.
Write endurance is, by definition, one event—but read endurance is excellent. E′ centres decay with a time constant of 2.3 million years at 20 °C, so a fulgurite is a practically immutable archive. The limiting factor is mechanical: the glass is porous and fractures under thermal cycling. Protecting a specimen therefore means keeping it at constant humidity. The Algerian sample is stored in a nitrogen-filled Pelican case with a silica-gel buffer; at 45 % RH the fracture propagation rate is calculated to be one micro-crack every 140 years, giving conservators plenty of time to extract additional slices as technology improves.
Forensic geographers are already brainstorming casework. A downed UAV whose memory card is missing could still be tracked if its crash coincided with lightning; search grids would shrink from hundreds of square kilometres to the 30 m radius of a single fulgurite. Smugglers who cross empty quarters at night might leave more than footprints—any bolt that hits within tens of metres locks their path into glass. Even climate science stands to gain: comparing GPS signal strength in fossil strikes against ionospheric models from the same date yields total electron content snapshots, retroactively validating space-weather algorithms.
Hardware is moving out of the synchrotron. A Portuguese start-up couples a 100 mW 1.5 GHz Gunn diode to a piezo stage small enough for field work. Powered from a car inverter, the unit can map a 20 cm tube in eight hours inside a tent. Data are uploaded over Starlink; cloud servers return position fixes overnight. The entire package fits in a backpack and costs less than a high-end DJI drone, meaning “lightning archaeology” may soon join magnetometry and LIDAR as a standard survey tool.
For data-recovery engineers who thought their world ended at silicon, fulgurites are a humbling reminder: the planet has been backing up radio signals in glass for millions of years—we just never had the decoder ring. Now that we do, every desert thunderstorm is a potential data-center breach, every glittering tube of frozen sand a drive waiting to be mounted. All that remains is to plug in the antenna, align the code, and listen to the desert tell you exactly where it stood when the sky caught fire.
