Modern satellite technology lets us spot dangerous threats like extreme weather, giant icebergs, and even foreign militaries. But when it comes to sinkholes, all we can do is wait and hope that our cars won’t be swallowed by a sudden gaping chasm in a city street. So how do sinkholes form, and why is it so hard to predict where they’ll appear?

YouTube’s Practical Engineering, also known as civil engineer Grady Hillhouse, built a miniature replica that reveals exactly what causes most sinkholes that appear in large cities. You’re probably more familiar with erosion as it applies to tides slowly washing away a beach, or causing cliffs along the coast to collapse. But any where that water can flow, there’s the risk of erosion creating catastrophic instabilities.

In the case of large cities, more often than not it’s the result of a pipe leaking, or completely bursting, underground. It can take just a few hours, or many months, depending on the rate of flow, but over time that liquid is going to displace enough sand or soil to create a large underground cavern that’s all but impossible to spot until it’s too late. Eventually, all that’s left is the road surface on top acting as an impromptu bridge that doesn’t stand a chance against the pull of gravity.

Unfortunately given the thousands of miles of piping hidden beneath cities around the world, it’s usually impossible to find a leak before it turns into an even bigger headache. So until we figure out how to design pipes that never break and adapt to shifting soil, or start laying all of our sewage and water pipes above ground, sinkholes will continue to be another one of the joys of living in a crowded urban center.