There was a time, before football and rock'n'roll, when explorers were the A-listers of their day. Death-defying antics and tales of daring made them the stuff of legend: names like Columbus, Raleigh, and Cook, who sailed off over the horizon to discover new lands and bring home treasures unimaginable to those sitting at home in dark, damp Europe.    Intrepid explorers they may have been, but ‘great' might be pushing it. Across the seven seas, they spilled blood and spread disease. They enabled the destruction of civilisations and the growth of slavery.  And many of their ‘discoveries' weren't quite what you'd think...