Why do people think the world will end in 2012?

ok, it seems to me as if every so often people need an excuse to get all up i arms about the world coming to an end. happened in 1999, remember Y2K? and lord knows how many christians thought the apocolipse was on the brink, anyways why 2012? whats so significant about it? and after that year pes whats "plan C"so to speak