The most common type of sinkhole
occurs when rainwater
d 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 collapse
s 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.