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A hole in the ground caused by rainwater eroding sand that was under the ground's surface. The empty space eventually collapses, much to the dismay of humans who were walking across or building on the surface at that location. Florida is prone to sinkholes because of the weather -- lots of rain for the erosion and some droughts which evaporate water from the ground and make the sandy areas take up less volume, weakening the support for anything built on that patch.

They can be sort-of fixed by filling the empty cavern (which may be much bigger than the actual collapsed area) with more sand held together with cement. However, to do so involves knowing where one has formed, which usually isn't discovered before it has collapsed. Sinkholes have "swallowed" whole houses, and one is as I write this forcing the closure of parts of two roads on the University of South Florida campus.

The most common type of sinkhole occurs when rainwater has dissolved a limerock substrate. The limerock is pretty strong, and it can hold up quite a bit of weight. When enough of the limerock has been eaten away, the top of the dome collapses without warning, swallowing anything that was on there at the time.

The second most common type of sinkhole occurs when a high-pressure waterline or high-volume drainline breaks. As water shoots out of the pipe underground, it eats a hole in the dirt. Because the pipe is hidden (and usually many decades old), nobody suspects until a large section above the pipe collapses. I recall a major pipebreak in San Diego, CA, which ate a large section of an onramp to highway I-8. Because it happened at night, a lady drove into it and was seriously injured. The folks behind her stopped and flagged down all the oncoming traffic. They ended up digging a 700ft ditch to get at the pipe.

Sinkhole is also techspeak for a DNS server which was designed to capture malicious traffic and prevent control of infected computers by the crackers who infected them with malware. Its job is simply to give out non-routable addresses, preventing malicious processes from accessing the target site.

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