KENEMA, SIERRA LEONE—The best athlete in the world, 26-year-old Umaru Conteh, whose natural talents would earn him unimaginable fortune as a global star had he been born into a society with a leisure economy sufficient to support professional sports, was on Saturday marched back into the mine where he is forced to work by armed guards."Go!" the AK-47-wielding man screamed at Conteh, whose one-in-a-billion combination of unbelievably dense fast-twitch muscle, otherworldly eye-hand coordination, and lightning reflexes would enable him to excel in any sport, despite his never having tasted clean water during his two decades of shackled labor in a poorly lit mine. "Down the shaft! Now!" After working for 18 consecutive hours to extract a tiny engagement-ring stone for a twice-divorced Tampa, FL real estate agent, Conteh was transported back home to the slum where he will die years before his unsuspected athletic abilities have a chance to decline.