The ancient ruins of Tiwanaku are  located in Bolivia, near Lake Titicaca, at an altitude of almost 13,000 feet (4,000 meters) above sea level, making it one of the highest urban centers ever constructed.  The city reached its peak between roughly A.D. 500 and A.D. 1000 and had at least 10,000 people living in it.  At this point only a small portion of the city has been excavated. 

Some remarkable artifacts have been discovered and examined at Tiahuanaco, not the least of which was a large monolithic Sun Gate which must have been an important part of the Kalasasaya, the chief temple of the ancient city. Its upper part is covered with an intricate sculpture that has been described as a calendar almost as long as the monolithic gateway. In fact, the Sun Gate has also been called the Calendar Gate. This calendar sculpture depicts a solar year, but not one that fits into the solar year as we know it.