Drones Found Giant Cracks Hidden Beneath an Icelandic Town — With Almost No Sign at the Surface

From above, parts of Grindavík looked almost normal.

Beneath the streets, playgrounds and football field, they weren’t.

During the extraordinary volcanic and tectonic crisis that has repeatedly shaken southwest Iceland since 2023, scientists began flying drones carrying sensitive magnetic instruments over the town.

What they found was unsettling: large fractures and cavities hidden underground that conventional surface mapping could barely see — or couldn’t see at all. These structures are dramatic examples of the fissures and Earth cracks that can form when tectonic or volcanic forces pull, fracture and deform the crust.

Some of the magnetic anomalies were later excavated.

The cracks were really there.

At one location beneath Túngata street, excavation revealed a hidden fracture roughly 2–3 metres wide beneath surface layers that had concealed it from view.

Elsewhere, beneath Grindavík’s football field, magnetic measurements detected substantial subsurface cavities even though the ground above showed limited or no obvious surface expression.

The findings provide a remarkable new look beneath a town that sits directly within Iceland’s reawakened Reykjanes volcanic and rift system.

Scientists Used Magnetic Drones to Look Beneath Grindavík

The research was published in Geophysical Research Letters by scientists led by E. J. Piispa of the University of Iceland.

Their goal was not simply to photograph cracks from the air.

They wanted to detect fractures that were invisible from above.

To do that, the team combined two types of magnetic surveying:

• UAV magnetometry, using a magnetic sensor suspended beneath a drone;
• ground magnetometry, with scientists carrying magnetic instruments across accessible areas.

In total, researchers collected approximately 220 km of drone survey profiles and another 100 km of ground-based magnetic measurements across Grindavík.

The drones normally flew only about 15–20 metres above the ground, with survey lines spaced roughly 10–15 metres apart.

In particularly dangerous or inaccessible areas, manually controlled drones were flown just 3–4 metres above the surface.

This allowed researchers to rapidly map parts of the fractured town that would have been unsafe to investigate on foot.

Why Do Underground Cracks Show Up Magnetically?

Grindavík is built largely on relatively young basaltic lava flows.

Those rocks are strongly magnetic.

But an open fracture isn’t solid basalt.

Neither is an underground cavity.

When a crack, void or fracture zone interrupts the magnetized rock, it can produce a localized reduction in the measured magnetic field.

On the researchers’ maps, many fractures therefore appeared as narrow linear magnetic lows.

More localized circular or semicircular magnetic lows could indicate shallow cavities or voids developing along those fractures.

These aren’t necessarily sinkholes themselves, but hidden underground voids are important because cavity growth and collapse are central processes behind many sinkholes and dolines.

A 2–3 Metre-Wide Fracture Was Hiding Beneath Túngata

One of the most striking examples came from Túngata street.

Surface mapping had identified no obvious fault trace across the location.

The magnetic data told a different story.

Two broad elongated magnetic lows crossed the area.

Researchers excavated one of them.

Beneath the apparently intact surface they found a fracture approximately 2–3 metres wide.

Layers of younger soil and lava above the fracture had effectively hidden the deeper deformation.

Without the magnetic anomaly, someone standing on the surface might never have realized what existed underneath.

Hidden Cavities Were Also Found Beneath a Football Field

Another dramatic case appeared at Grindavík’s football field.

Drone and ground surveys detected a broad linear magnetic anomaly together with a strong, semi-circular magnetic low interpreted as a potential shallow cavity.

Yet photographs of the field showed little to no surface expression of the large voids beneath it.

Excavation later exposed one of the fractures.

The discovery illustrates the central hazard revealed by the study:

The absence of a dramatic crack at the surface does not necessarily mean the ground beneath it is intact.

Some Grindavík Fractures Reached About 3 Metres Wide

The overall deformation documented across Grindavík was enormous.

Within the town, researchers report fractures reaching approximately 3 metres in width, with vertical offsets of up to about 1 metre.

Fractures and voids have been measured to depths of around 30 metres.

That does not mean the drone system directly mapped every fracture down to 30 metres.

Magnetic modelling indicates that the anomalies detected in the survey are most consistent with fractures and cavities primarily within the upper 10–20 metres of bedrock.

Even that shallow zone is critically important because it is where roads, houses, pipes and other urban infrastructure are anchored.

Why Is Grindavík Breaking Apart?

The hidden fractures are part of a much larger geological transformation underway beneath southwest Iceland.

Since November 2023, repeated magma intrusions beneath and around Grindavík have forced the crust apart, producing extensive faulting, fracturing and land subsidence.

Two grabens — elongated blocks of crust that drop downward between faults — formed across the area, showing how volcanic rifting can produce both dramatic horizontal cracking and vertical ground displacement.

These processes are linked to the renewed activity of the Reykjanes Peninsula, where repeated dyke intrusions and fissure eruptions have marked the return of intense volcanic activity after centuries of relative quiet.

You can explore the wider geological setting in our guide to Iceland volcanoes, rift zones and fissure eruptions.

Magma moving underground does not need to erupt at the surface to cause serious damage.

A dyke intrusion can physically force the crust apart, reactivate older fractures, produce earthquakes and create zones of intense ground deformation. Active volcanic systems can also release gases through fractures and permeable ground, making Earth degassing and toxic gas emissions another important hazard in volcanically active regions.

Together, ground deformation, fissuring, gas release and eruptive activity form part of the wider range of volcanic hazards that must be monitored during an evolving volcano-tectonic crisis.

Some “New” Cracks Were Actually Old Fractures Reactivated

The magnetic maps revealed another fascinating detail.

Several anomalies aligned with fractures visible in historical aerial photographs.

In other words, some structures beneath Grindavík were not necessarily created from scratch during the latest crisis.

Older weaknesses in the crust appear to have been reactivated by the renewed rifting.

That matters for volcanic and earthquake hazard assessment because the subsurface can preserve structural weaknesses long after their obvious surface expression has disappeared.

Future tectonic movement may then exploit those same weaknesses again.

The Maps Could Be Produced Within Days of a New Event

Perhaps the most important part of the research is that this was not simply a geological reconstruction performed years later.

The magnetic surveys were conducted during an active volcano-tectonic emergency.

According to the researchers, the combined workflow could provide actionable information about subsurface hazards to civil-protection authorities within days after a major event.

The maps revealed fracture continuity, branching patterns and changing fracture widths, including in areas too dangerous for conventional ground surveys.

That information directly supported hazard assessment and emergency response.

It adds another potentially powerful technique to the collection of tools scientists already use for volcano monitoring and eruption forecasting, alongside earthquake monitoring, GPS deformation measurements, satellite radar and gas observations.

A New Way to Map Hidden Ground Hazards During a Volcanic Crisis

The researchers describe the Grindavík work as the first combined UAV- and ground-based magnetometry application for near-real-time fracture mapping inside an inhabited area during an active volcano-tectonic crisis.

That doesn’t mean the method will work everywhere.

Grindavík is unusually well suited to magnetic surveying because its young basaltic rocks are strongly magnetized.

The technique depends on having enough magnetic contrast between intact rock and the fractures, voids or material filling them.

Dense infrastructure can also complicate magnetic measurements because buildings, utilities and other human-made objects generate their own magnetic anomalies.

In geologically complex areas with weaker magnetic contrasts, researchers may need other geophysical techniques.

But in volcanic regions dominated by strongly magnetic rocks, the approach could become a rapid way to identify dangerous subsurface deformation before it becomes obvious at the surface.

What You See at the Surface Isn’t Always What’s Beneath Your Feet

Grindavík has already provided some of the most extraordinary images of the renewed Icelandic volcanic crisis: roads split apart, steaming fissures cutting through town and lava advancing toward buildings.

But this study reveals another, less visible side of the disaster.

Some of the ground can appear intact while metre-wide fractures and cavities are hiding underneath.

That’s why the magnetic surveys are so compelling.

They turned an invisible geological hazard into something scientists could actually map.

And in a town repeatedly shaken by earthquakes, magma intrusions and ground deformation, knowing what lies beneath the surface can be just as important as watching the eruption itself.