Why do sinkholes form in urban areas?

A: Sinkholes appear in urban areas when the ground beneath streets, buildings or pipelines suddenly gives way. They can form naturally but in cities they’re often linked to human activity.

Many cities are built on karst terrain, where the bedrock, often limestone, gypsum or salt, dissolves in water. Over time, underground cavities form as rainwater or leaking pipes seep downward, dissolving the rock. Eventually, the ceiling of these cavities collapses, creating a sinkhole on the surface. Old or broken water mains and sewer lines can leak for years, washing away soil and creating hidden voids. 

Road traffic, building foundations, and underground tunnelling can also put extra pressure on the ground. If the soil beneath has already been weakened by erosion or cavities, these added loads can trigger a sudden collapse. Intense rainfall or flooding, due to poor urban planning, can also rapidly infiltrate soils, especially sandy or loose ones, and accelerate erosion.

Paving over large areas without proper drainage or building in geologically unstable zones can also increase sinkhole risk. Urban expansion sometimes ignores the natural characteristics of the land and sets the stage for collapses to occur later.