Ancient Aliens

If ancient aliens visited Earth, what was their mission, and is there evidence that points to when they will return? Ancient Sumerian tablets describe an alien race, known as the Anunnaki, who came to Earth to mine for gold. Egyptian hieroglyphs depict hybrid creatures that were part man, part animal. Mysterious crop circles and unexplainable crystal skulls are thought to contain messages that man has yet to decipher. Strange evidence may exist in multiple cultures throughout history suggesting that aliens came here to explore, excavate raw materials, experiment on human beings, and even for pure conquest.