There is zero scientific evidence that anything will happen, but there are numerous things that could easily threaten the human species in the year 2012, such as a geomagnetic reversal, asteroid strike or supernova. Or soemthing more intimate, like a flu pandemic, or a nuclear war.
Atleast that's what it says here,
http://www.survive2012.com/
I for one doubt the earth will get hit by an asteroid anytime soon. If I remember correctly I think there has been an asteroid detected that will pass real close to us but this won't occur til 2028 I think. The only thing that could happen is some strange geomagnetic reversal. Others think planet X / Nibiru will make a re-appearance and cause strange phenomenons around Earth.
A lot of things out there that talk about this day. I've done my research on it and find this stuff somewhat religious like. Not really but kinda... Here's a conspiracy theory
http://www.2012endofdays.org/more/Conspiracy-Theory.php and here's a video that talks about Nibiru and other shit.
http://www.wat.tv/video/2012-illumin...7ip_vzfh_.html this is a video with 7 parts... there are tons of other videos out there that talks about 2012. Some talk about aliens and creators. Any of you seen Indian Jones and crystal skull? Notice how the alien mother ship was inside one of the temples? Well, there is one video that I've seen out there that talks about some movies that have been made by "current" flim makers that are "trying" to send some kind of sign out there. To warn us or something.
Seriously though, I think the people that will take this as a life or death situation will be the millions of parents around the world because their kids Christmas is canceled and they will probably end up being the reason why we enter the "end up days." lmfao, fucking kids.
ETA: The correct date is 12/12/2012. It's not the 21st.