Sinkhole [Physical Geography]
Noun. A sinkhole is a hole in the ground that forms when water dissolves surface rock. Often, this surface rock is limestone, which is easily eroded, or worn away, by the movement of water.

In a landscape where limestone sits underneath the soil, water from rainfall collects in cracks in the stone. These cracks are called joints. Slowly, as the limestone dissolves and is carried away, the joints widen until the ground above them becomes unstable and collapses. The collapse often happens very suddenly without warning. Water collects in these collapsed sections, forming sinkholes.

Sinkholes also form when the roofs of caves collapse. Sinkholes are often funnel-shaped, with the wide end open at the surface and the narrow end at the bottom of the pool. Sinkholes vary from shallow holes about 1 meter (3 feet) deep, to pits more than 50 meters (165 feet) deep. Water can drain through a sinkhole into an underground channel or a cave. When mud or debris plugs one of these underground caves, it fills with water to become a lake or a pond. (National Geographic Education)

Although sinkholes are created over a long period of time, when they
finally appear on the landscape, it can come as quite a surprise. Take
what happened in Daisetta, Texas for example. Do you think the residents
of Daisetta could have been better prepared for this natural disaster?
What would you do if a natural disaster like this one occurred in your
town?  

When it comes to unusual geography, we at National Geographic Education
believe that is it always better to be safe than sorry.  To learn more
on the subject and to prepare yourself and your students for unexpected
geography in action, check out our extreme natural disaster classroom
activity, where you will find resources and tips to help you ward off
any stormy surprise!  

Photo Credits: Harry Berking (Your Shot), Ilan Shacham (Your Shot)
–Julia from My Wonderful World