From the USA.gov Blog:
False rumors about the end of the world in 2012 have been commonplace on the Internet for some time. Many of these rumors involve the Mayan calendar ending in 2012 (it won’t), a comet causing catastrophic effects (definitely not), a hidden planet sneaking up and colliding with us (no and no), and many others.
The world will not end on December 21, 2012, or any day in 2012.
Unfortunately, these rumors have many people frightened, especially children. NASA has received thousands of letters concerned about the end of the world. David Morrison, a planetary astronomer and senior scientist for NASA who answers questions from the public about astrobiology, says, “At least a once a week I get a message from a young person ― as young as 11 ― who says they are ill and/or contemplating suicide because of the coming doomsday.”
According to NASA, the old mystery-planet-collision rumor year was 2003, but when 2004 arrived safely, the rumors changed to 2012. So what end-of-the-world year will the rumor mill make up next?
Videos from NASA debunking end-of-the-world rumors: