Icelandic Town Hides Massive Invisible Underground Fractures Revealed by Drones
Scientists using magnetic drones discovered massive underground fractures and sinkholes beneath Grindavík in Iceland. The invisible hazards threaten infrastructure following volcanic activity.
From above, large parts of the town of Grindavík in southwestern Iceland appeared completely intact and normal, but beneath the streets, pathways, and the municipal soccer field, the reality is far more dangerous. During the volcanic and tectonic crisis that has been shaking the region since late 2023, scientists operated drones equipped with sensitive magnetic sensors over the town. The findings, published in the scientific journal Geophysical Research Letters, revealed massive fractures and dangerous underground cavities hiding beneath seemingly innocent surfaces—an invisible threat that traditional mapping methods failed to detect.
A team of researchers, led by A. J. Piske from the University of Iceland, set out to map the subsurface in areas where walking on foot had become too dangerous. To achieve this, the researchers combined two magnetic measurement methods: scanning using drones that flew at an altitude of only 15 to 20 meters, and in particularly dangerous areas at just 3 to 4 meters above the ground, along with manual ground measurements in accessible areas. In total, data was collected along approximately 220 kilometers of drone flights and about 100 kilometers of pedestrian measurements.
The method was based on the geological characteristics of the region: Grindavík is built upon young basalt volcanic layers with strong magnetism. When an open crack, fissure, or empty cavity forms in the rock, the local magnetic field drops. The measurement maps translated these drops into clear drawings of elongated fractures and sinkhole-like circular cavities deep in the ground.
To verify the magnetic instrument data, the researchers conducted controlled excavations at several points found to be anomalous. The results fully confirmed the fear: beneath Tungata Street, where the surface appeared smooth and without any external cracks, a fracture 2 to 3 meters wide was exposed at depth, completely concealed by upper soil and lava layers.
Another alarming discovery was located beneath the town's soccer field. While the grass surface appeared completely normal with no visible subsidence, the scans identified a magnetic anomaly indicating a large-scale underground cavity, which was later revealed by excavation. The researchers emphasize that the absence of a visible crack on the surface in no way indicates the stability of the infrastructure beneath it.
In total, fractures up to about 3 meters wide were documented across the town, with vertical displacements of up to 1 meter, some reaching depths of tens of meters. The magnetic models showed that most of the dangerous cavities are concentrated in the upper 10 to 20 meters of the bedrock—the critical layer upon which roads, homes, municipal pipelines, and public buildings rest.
These fractures are the result of intensive tectonic and volcanic activity on the Reykjanes Peninsula, where magma intrusions push and fracture the Earth's crust without requiring a visible surface lava eruption. This activity creates ground depressions (grabens), earthquakes, and toxic gas emissions. The new mapping also revealed that some of the "new fractures" are actually ancient faults that cracked in the past and have now reopened due to tectonic pressures.
The primary significance of the research lies in its ability to provide rapid operational information: instead of waiting months for geological analysis, the system allowed precise collapse hazard maps to be delivered to civil defense authorities within just a few days following a volcanic event, thereby preventing disasters in populated areas prone to volcanic activity.
