aztec 2012 thingy (im not like astromy or whatever)

Ok but why did the aztecs think the world was gonna end in 2012?? like why were people taking that seriously like that was 500+ years ago! (i know most people werent really but its still funny to me) like i dont think a black hole is gonna swallow us all up (maybe in a billion years :o ) but how would some ancient kingdoms predict that ;-; 

also feel free to tell me why they thought that in the comments ;-)