Did you know - Sinkholes.

[JennyLeez]

Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by ground water circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small, as this picture shows, or they can be huge and can occur where a house or road is on top.
USGS

In Soviet Russia, the ground moves you. Berezniki’s sinkhole began in 1986 and just grows worse with each passing year. It’s unstoppable. Currently it’s over 200m deep, 80m long and 40m wide. In case you’re thinking “Berezniki? Who cares?” you should know that 10% of the world output of potash comes from this area, and the sinkhole is very close to destroying the mine’s sole rail line.


Berezniki - one of the largest in the world.

Residents of a Guatemala City heard strange rumblings for weeks but weren’t sure what was happening beneath them. Then, in late February 2007, a near-perfect circle of earth dropped some 30 stories almost instantly. It’s amazing how neat the hole is. Two people died and over 1,000 had to be evacuated; the sinkhole resulted from a corroded sewage system deep beneath the surface (apparently the odor coming from the hole was intolerable).


Guatemala City - also one of the world's largest.

13 of the Biggest, Strangest, and Most Devastating Sinkholes on Earth can be found here:
http://webecoist.momtastic.com/2008/08/26/incredible-strange-amazing-sinkholes/

In New Zealand the Waitomo Caves are also referred to as sinkholes as the river vanishes into the hillside. The caves take their name from wai ( water) tomo (cave ).

The Harwoods Hole, Takaka Hill is the deepest vertical shaft sinkhole in the Southern Hemisphere. It is 357 metres in Height and 50 metres Opening.


Takaka Hill, between Golden Bay and Tasman Bay, New Zealand.

Spectacular sinkholes are a feature of the downlands at Craigmore, south-west of Timaru. The hills are porous limestone, which corrodes when rain filters down into it. This causes the surface to sink, creating holes.


Craigmore, Maungati, near Timaru, New Zealand

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[Tony]

I didn't know about the Craigmore sink holes. I might go and see if i can check these out.

[Sara]

Thanks. Makes interesting reading.