CNN  —  Six tourists wearing hard hats and heavy overalls sit, cramped up in a narrow mineshaft, with barely enough space to kneel. The local tour guide pulls out a disposable lighter, ignites a bright green fuse with it, and calmly ushers everyone backward. “Any moment,” he says. A moment later, a powerful shockwave rips through the tunnel, tailed by a dust cloud. He’s just set off a stick of dynamite bought at the local market earlier that day by one of the tourists — it cost 13 Bolivianos (