This is one of the most impressive ruins found on the planet.
A few silly hypotheses:
1. Extraterrestrial Alien help
2. The human people worked for a long..long.. time to accomplish this feat
3. Humans built it using a higher technology and were later almost completely wiped out through some type of cataclysm. There is a theory that the evolution of human life has been through many cycles.
4. Time travelers from the future
5. Any other ideas?