For almost as long as we've had civilization, we've lost it. There are records going back hundreds of years of explorers discovering huge temples encrusted with jungle, or giant pits full of treasure that were once grand palaces. Why did people abandon these once-thriving cities, agricultural centers, and trade routes? Often, the answer is unknown. Here are ten great civilizations whose demise remains a mystery. 1. The Maya The Maya are perhaps the classic example of a civilization that was completely lost, its great monuments, cities and roads swallowed up by the central American jungles, and its peoples scattered to...