I felt sorry for this OAP's "shock garden"
Apparently an old collapsed mine shaft that goes down 800 ft, in Cumbria.
(http://img140.echo.cx/img140/669/02005290760002yr.jpg) (http://www.imageshack.us)
No need to dig that pond they always wanted!
Lucky things though - looks really good dark loamy soil!
I think the whole street had to be evacuated, as there are disused mine shafts under the lot.
I was once on a geology field trip in Wales somewhere, and I chucked a stone down a 700-foot mineshaft. The way it went on and on banging off the walls was quite something. Mostly when they were abandoned they just boarded them over. Then the boards got covered in soil and plants, and the boards rotted. There are places in Cornwall where they know there are mineshafts, but don't know exactly where; you're definitely not recommended to wander off the paths!