7 things you might not know about sinkholes

Highway 83

Folks in urban areas are probably familiar with the sinkhole phenomenon—one just appeared at a busy intersection in Toronto this week—but what Canadians may not know is that sinkholes are a lot more common than they might think, especially in their own country. One of the most dramatic sinkholes to appear in Canada happened along a Manitoba highway in 2012. Twelve inches of precipitation caused approximately 200 metres of Highway 83 near Inglis to wash away. At certain points, the ground sat eight metres below its original surface point.

Whether they’re swallowing cars whole or creating opportunities for spelunkers, sinkholes are fascinating examples of geology gone to the extreme.

1. Urban sinkholes are usually caused by water main breaks or faulty sewer lines—and aren’t technically sinkholes. When an old sewer pipe or water main gives way, the street above follows suit. For the most part, these urban sinkholes are more inconvenient than anything—though one four-metre sinkhole in Montreal that formed after a major student protest had walked over it could have been disastrous if it had opened up during the protest.

2. Sinkholes usually involve specific types of rock. Sinkholes often form when the bedrock below the land surface is porous or slightly soluble. Bedrock made of limestone, gypsum, sandstone or salt can be dissolved by percolating groundwater, making it unstable.

3. Natural sinkholes are formed because of activity below the earth. The erosion of bedrock  can cause the surface ground to sink, as can the collapse of an underground cave roof or the shifting of the water table.

4. Not all sinkholes are on land. Called blue holes, underwater sinkholes are scenically incredible. One of the most amazing is the Great Blue Hole in Belize, which was formed underwater, measures more than 300 metres across, and is more than 124 metres deep. The deepest, Dean’s Blue Hole in the Bahamas, plunges to a depth of 202 metres.

5. Sinkholes are common features of karst landscapes. Karsts are formed by the dissolution of soluble rocks like limestone, dolomite, and gypsum, forming sinkholes and caves that have underground drainage systems. The Eramosa Karst, on the Niagara Escarpment in Southern Ontario, has several classic karst features, including sinkholes.

6. Some sinkholes double as swimming holes. The appetizingly named Devil’s Toilet Bowl in Florida (also known as Devil’s Hole) and any number of formations called cenotes in Mexico are prime swimming and snorkelling spots. Some cenotes offer snorkelling in near light-free conditions.

7. The world’s largest sinkhole is in China, and it’s heavenly. Called Xiaozhai Tiankeng—or Heavenly Pit—the world’s largest sinkhole is 626 metres long, 537 metres, wide and 662 metres deep