They appear with no warning and cause no end of problems. In the past fortnight we've seen at least eight sink holes in the South East.
Last week the M2 was closed for hours as a crater appeared next to the carriageway.
On Sunday half of a street in Watford was evacuated when a 20 foot deep chasm opened
Sink holes are caused when the underlying rock is washed away - more exactly by the acid picked up by rain water as it passes through rotting vegetation. That creates cavities underground which streams can flow through - the more water that flows the quicker the erosion
It's the sheer amount of rain water which has accelerated the damage - and we're not out of the woods yet. With two weeks left till the end of February we've already seen the wettest winter since records began back in 1910. We may be feeling the effects for many more weeks to come.
See Martin's report on sink holes in the south east.
Watch this video from the British Geological Survey on how sinkholes form.