Scientists discover a surprising new way to map what lies beneath cities using thunder, revealing hidden underground hazards and possible sinkhole zones

Thunderstorms may offer more than spectacular flashes of lightning and booming skies. Scientists have found that the powerful vibrations produced by thunder can be used to map underground geology, potentially helping identify hidden hazards such as weak rock zones and areas vulnerable to sinkholes.

Researchers at Pennsylvania State University used existing underground fibre-optic cables beneath the University Park campus to detect tiny ground movements triggered by thunderstorms. By analysing the signals from hundreds of these events, the team produced detailed images of the shallow subsurface.

The findings, published in Science Advances, point to a potentially cheaper way of carrying out seismic surveys, particularly in cities where extensive fibre-optic networks are already buried underground.

Thunder creates tiny earthquakes beneath the surface

Thunder generates powerful acoustic waves that travel through the atmosphere. Some of that energy reaches the ground, creating minute vibrations that are far too weak for people to feel.

Scientists have now shown that these vibrations can nevertheless be detected using fibre-optic cables.

The Penn State team, led by geophysicist Tieyuan Zhu, analysed two and a half years of seismic data collected from fibre-optic cables beneath the University Park campus. The dataset included signals from more than 450 thunderstorms or thunder-generated seismic events, sometimes referred to as “thunderquakes”.

The technique relies on distributed acoustic sensing, which turns ordinary fibre-optic cables into a dense network of seismic sensors.

How fibre-optic cables detect underground movement

The cables were originally installed for telecommunications and stretch for roughly four kilometres beneath State College, Pennsylvania, at a depth of about one metre.

Researchers have increasingly explored the use of unused or “dark” fibre networks for monitoring earthquakes and other ground movements.

When a thunder-generated vibration passes through the ground, it causes the buried fibre to stretch by an extremely small amount. That changes the time taken for light to travel through the cable.

The change is tiny, but sensitive instruments can detect it and convert it into information about ground motion.

The principle is similar to how a conventional seismograph records vibrations caused by an earthquake, although the sensing system uses the fibre itself as a distributed detector.

Thunderquakes reveal what lies beneath the ground

The earlier Penn State research established that fibre-optic cables could pick up seismic signals generated by thunder. The latest work takes the idea further.

By combining signals from numerous thunderquakes, researchers were able to create images of underground structures extending to a depth of roughly 100 metres.

That proved particularly useful at Penn State because the campus sits on a karst landscape, where limestone has been shaped by underground water over long periods. Such terrain can contain caves, fractures and other weaknesses that may pose risks to buildings and infrastructure.

The new underground images revealed several areas of potentially weakened limestone that had not previously been detected.

Researchers cross-checked possible sinkhole zones

The scientists did not rely solely on the thunderquake data.

They compared the underground images with information from boreholes drilled at the site. They also examined satellite measurements showing subtle changes in the shape of the ground.

The independent observations supported the presence of several of the weak zones identified by the seismic analysis.

That combination of data is important because underground cavities and fractured rock can be difficult to identify from the surface until they begin causing problems.

Could thunderstorms replace expensive seismic surveys?

Traditional seismic surveys can require specialised equipment, significant manpower and controlled sources of seismic energy.

Thunderstorms, by contrast, provide a naturally occurring source of strong vibrations.

A single thunderstorm can generate signals capable of covering tens of square kilometres, according to the researchers. That raises the possibility of using storms as a large-scale natural source for studying shallow underground structures.

The approach could be particularly attractive in urban areas, where existing fibre-optic networks could potentially provide extensive coverage without requiring researchers to install a completely new array of sensors.

Scientists say this could eventually make subsurface surveys faster or less expensive in some locations.

The idea could extend beyond Earth

The researchers are also considering whether naturally generated seismic signals could be useful beyond our planet.

One possibility is Titan, Saturn's largest moon. Titan is known for its thick atmosphere and powerful, long-lasting storms.

If suitable seismic sensing technology could be deployed there, storm-generated vibrations might provide another way to investigate the moon's hidden interior.

For now, however, the most immediate potential lies much closer to home.

Turning existing telecommunications infrastructure into a vast underground sensing network could give scientists a new way to study cities and identify geological hazards before they become visible at the surface.

The research suggests that the next time thunder rolls across a city, it may be doing more than announcing a storm—it could also be sending scientists a faint signal from the ground below.