If you ever plan to travel between North America and South America, you must consider that there is no road. Ten hours southeast of the Panama Canal, the Pan-American Highway penetrates the jungle, shrivels into a footpath and dies. The highway resurrects in Colombia, another continent. But the land between the two countries is a vast and primitive realm. On a map the two ends of the highway appear as two slivers of life, separated by the unknown. Filling this void is a rugged wilderness known as the Darien Rainforest. Because the Darien hinders all contact by land between North America and...