Throughout ancient literature, and the ancient world, there are literary references to, and artifacts which virtually prove that at least one group of extraterrestrial beings has visited Earth. The Book of Ezekiel in the Bible speaks of the Celestial Chariot (that's a fancy way to say space ship). The Book of Genesis speaks of a group of people called the Nephilim. The word Nephilim means "Those who from heaven to Earth came". Genesis 1:26 reads: Then God said, "Let Us make man in Our Image..." I submit to you that the 'god' referred to was what we would now call an ET.
The Ancient Sumerian culture of the Middle East show their god Ashur in a disc with wings. They also have stone carvings that show the genetic double helix. Sumerian artifacts date at least as far back as 3000 BC. The Sumerians also have scale models of our solar system that date back to the same period.
In these star maps, the Sumerians show 12 planets including our sun and moon. One of which is Pluto. This celestial object (no longer considered a planet by modern astronomers) was not discovered by our scientists until 1930. How could the Sumerians have known of it's existence thousands of years before we did? Perhaps we should take their stellar expertise more seriously.
The other Planet that the Sumerians claim to have knowledge of, is one known as Nibiru. This is where they claim their gods came from. The Sumerian historical record also indicates that they believe humans to have been genetically manufactured by the race of beings that are said to inhabit this planet called Nibiru. This race of beings is know to the Sumerians as the Anunnaki.
The Sumerians have described the Anunnaki (also believed to be the Nephilim referred to in 'Genesis') as a gigantic race of beings. This would easily explain the construction of some of the colossal structures of ancient times, such as the Sumerian Ziggurats - which were the forerunners of the Egyptian pyramids and the Mayan Temples.
According to Sumerian celestial facts, The Anunnaki - residing on their planet, Nibiru - orbit the sun on an elliptical path that takes 3600 earth years to complete. Every time the orbit of Nibiru enters into the stages of its solar perigee (or closest point to the sun), its inhabitants (the Anunnaki) are able to traverse the short remaining distance to Earth. The sumerians claim that it is during those times - every 3600 years - that the Earth has been visited by their gods. And that furthermore, it was during one such period that mankind as we know it was bioengineered by the Anunnaki. These claims are substantiated and further explained by well known scientists and researchers such as Zecharia Sitchin and Erich Von Daniken.
The next time that Earth is supposed to be visited by The Anunnaki, or at least a close pass from Nibiru is December 21, 2012.