After record rainfall fell on New Zealand’s North Island in May 2018, a farm worker noticed that a sinkhole had formed overnight – and yet he couldn’t quite see its size until the sun came up. Then, with the light of day, the worker realized that the Earth had cracked open, creating a fissure that stretched for hundreds of feet and swallowed up pasture roamed by cows.
Sinkholes that make headlines seem to open instantly, but some can actually take thousands of years to form. They commonly appear in areas that have limestone within their terrain. The sedimentary rock is, you see, easily infiltrated and dissolved by rainwater that becomes acidic after percolating through soil – and gathering a bit of carbon dioxide.
The absorbed carbon dioxide makes rainwater react with decaying plants, and it’s this process that transforms runoff into acidic water. As the liquid subsequently makes its way to the limestone layer, it fills the rock’s cracks and very slowly begins to dissolve it. So, over time, cavities in the limestone layer become larger and larger. And a sinkhole then occurs when the land above these cavities gives way, falling into the depression below.