A long standing Nautical Legend - perhaps attributed to those dreaded ''Wolves of The Sea'' - Pirates, is that of The ''Devil's Thumb''. Navigation during the ''Golden Age'' of Sail depended on ''good maps''. After Centuries of Seafaring, some fairly good one's were produced by the 17th Century AD. To be added, a skilled ''Pilot'' (or Navigator) was another real need. But as One can find, The Sea is a constantly changing locale. Hence, the belief of ''The Thumb''. In short, seemingly out of nowhere, a ship would ''run afoul'' of mysterious ''chunks of rock'' - which would ''magically appear'', when maps revealed no such anomaly.
It was ''thought'' that The Devil Himself would ''stick up'' His thumb (from The Sea) at low tides. Such rock dangers would be ''just barely seen'' and could be avoided. BUT - if ''Old Scratch'' lowered His digit (so as to NOT be seen) - a Ship could (and often would), have their hull ''ripped to shreds''. This meant death (drowning) and loss of vessel. The BEST CASE scenario? Slow starvation die to becoming ''floundered'' on Satan's Thumb! Of course, some ''Legends'' are just that.......
