According to Sumerian cuneiform stone tablets and writings that are 6,000 years old, Nibiru is known as the Planet of the crossing. Nibiru is also believed to be what Astronomers call Planet X
(the unknown 10th planet exerting forces in our solar system on our outer planets).
According to ancient Sumerian literature, the celestial object Nibiru travels once around our Sun every 3,600 years. It's orbit crosses the orbital path of Earth which means it's subject to causing catastrophic weather-related events on Earth. Nibiru is the home of the Sumerian Gods who are called the Anunnaki
, which is translated to mean "those who from heaven to earth came".
Famous author and linguist of many ancient languages, Zecharia Sitchin, refers to Nibiru as The 12th Planet. According to translations of ancient cuneiform writing, the Sumerians counted all of our 9 Solar System planets, plus counted the Sun and Moon as planets 10 and 11, and leaving Nibiru as the 12th Planet (home of the Gods).
Jason Martell has also contributed large amounts of time and travel on the subject of Nibiru and the Alien Astronaut Theory as well. He runs some very impressive websites which are loaded with large amounts of information pertaining to the Sumerian culture. Jason also has the largest Sumerian Artifact gallery on the internet available for public viewing.
There is also a spy-like PC game called "NiBiRu: Age Of Secrets" which is based on German sourced 12th Planet information, but is twisted a little to give more credit to the Mayan Civilization Secrets.
For more detailed information run a search for "Jason Martell Research". It will point you in the right direction.