Tiwanaku, Bolivia. - stock photo

Located in Bolivia, near Lake Titicaca, the ancient city of Tiwanaku was built almost 13,000 feet above sea level, making it one of the highest urban centers ever constructed. Surrounded, in large part, by mountains and hills, the city reached its peak between roughly A.D. 500 and A.D. 1000, growing to encompass an area of more than two square miles, organized in a grid plan. Only a small portion of the city has been excavated. Population estimates vary but at its peak Tiwanaku appears to have had at least 10,000 people living in it. Although its inhabitants didn’t develop a writing system, and its ancient name is unknown, archaeological remains indicate that the city’s cultural and political influence was felt across the southern Andes stretching into modern-day Peru, Chile and Argentina.