There were a lot of abandoned mineshafts around where I grew up in the north of England - chilling vertical holes, lined with millstone grit and quite often completely unprotected by the usual fences and skull and crossbones signs.
Being of a somewhat philosophical frame of mind, I found it difficult to resist peering cautiously down into the inky depths while terrifying myself with the thought of how very easy it would be to pitch myself in. Far better to lob down a sizeable rock and count the seconds before it hit the bottom with echoes either of deep water or the sharp crack of shattered stone. The depth of the shaft was then readily calculated by means of the familiar formula: depth (in feet) equals 16 times the delay (in seconds) squared.
Except that occasionally there was no sound from the bottom but only a succession of ever feinter scrapes as the plunging rock grazed the shaft walls. The inescapable conclusion was that these were bottomless pits and it was a good idea to move on and, above all, to resist any further thoughts of having a second look down.
Come to think of it, bottomless pits seemed to feature quite strongly in my boyish imagination. Of course, the real explanation was that the stone had simply thudded softly and inaudibly into the pile of dead sheep and old mattresses at the bottom of the hole.
Anyone who's tried writing a blog will immediately know what I'm talking about ...