The Hidden Danger Beneath Our Roads: How Underground Mines Cause Sinkholes and Subsidence

In December 2024 a massive sinkhole opened on I-80 near Wharton, New Jersey, with repeat events in 2025. Practical Engineering host Grady Hillhouse uses a garage-built room-and-pillar model to explain how underground mines deform the surface over time, why failures are often non-linear, and how water, geology, and abandonment contribute to subsidence. The video also covers historical mining practices, regulatory gaps, real-world examples, measurement methods, and remediation strategies, highlighting the social and policy dimensions of mine subsidence and the ongoing challenge of balancing resource extraction with public safety.

Introduction: a real world problem behind sinkholes

In December 2024 a huge sinkhole appeared in the center of a sports complex on I-80 near Wharton, New Jersey, followed by further collapses in February and March 2025. Grady Hillhouse of Practical Engineering presents a narrative that these events are not random or purely natural phenomena, but consequences of underground mining and subsidence. The video frames these incidents within a long arc of mining history and the modern challenges of keeping a major highway safe while extracting essential materials.

The garage model: a tangible demonstration of complexity

To illuminate the problem, Hillhouse builds a small room-and-pillar coal seam in his garage. The model uses cardboard to stand in for a coal seam, colored for visibility, and a rainfall simulator to mimic natural time progression. As sand and soil in the model are wetted and dried, surface soil sinks and, when the water reaches the mine void, the subsurface deforms in non-linear ways. The demonstration emphasizes that subsidence does not always progress smoothly; rather, it can progress in fits and starts, with small roof failures gradually propagating into larger surface deformations.

Mining methods and subsidence mechanisms

The video explains room-and-pillar mining as a traditional approach where the ore is extracted but columns of rock are left to support the roof. Pillar size and spacing reflect the strength of the material and the economics of resource recovery. A contrasting method, longwall mining, uses hydraulic supports that advance along the seam and cause intentional roof collapse behind them, which is highly efficient but increases surface subsidence risk. Hillhouse stresses that deeper mines create broader, more diffuse subsidence effects, and that deformation can extend well beyond the immediate footprint of the mine itself.

Historical context and regulatory landscape

The earliest mining eras in the United States had lax long term regulation, with safety rules focused on workers rather than surface stability. Property ownership complicates matters further when surface rights and mineral rights are held by different entities, limiting homeowners' control over what happens beneath their land. Abandoned mines commonly flood, dissolving minerals, softening soils, and eroding supporting structures. The video also discusses policy tools such as state insurance pools in states with heavy coal mining, and the federal Mining Act of 1977, which aimed to prevent subsidence and fund reclamation for mines abandoned before the act. Modern mining practices like longwall mining illustrate how engineering choices directly influence surface stability and risk exposure.

Real-world examples and measurement

As the host notes, there are iconic cases such as the 2024 Alton Illinois incident where an active underground aggregate mine below a park collapsed, producing a surface hole. Subsurface movement can be difficult to predict because surface effects depend on geology, depth, hydrology, and historical extraction. Engineers use inclinometers and extensometers to monitor surface movements and test predictions against observed subsidence. The video underlines the challenge of forecasting subsidence, which blends empirical data with geotechnical understanding and numerical modeling.

Remediation and policy implications

Remediation strategies range from backfilling voids with layered materials to prevent ongoing erosion, to grout injections or polyurethane foam for immediate stabilization, to structural retrofits for buildings above mines. The discussion emphasizes that while mining has become safer and more responsible over time, subsidence remains a persistent challenge that intersects land use, property rights, and environmental protection. The video argues for continued balancing of mineral resource needs with the protection of property and ecosystems, and it points to the broader community and policy dimensions of mine subsidence as an ongoing engineering and governance problem.

Closing notes: broader context and call to curiosity

The presentation ends with reflections on how the mining sector has evolved, and how public understanding of infrastructure hazards benefits from clear, evidence-based explanations. The video closes by highlighting related science content from the channel and the broader ecosystem of independent science media, including a nod to collaborations with other creators and platforms.