Reactor 1's core may have melted all the way through the concrete by now. Reports of cracks in the surrounding area with steam coming out of them indicates it may be hitting small amounts of moisture in the ground, outside of the containment.

If it hits a groundwater source, it could cause a hydrovolcanic explosion much worse than anything seen so far...
http://emsnews.wordpress.com/2011/12/01/fukushima-enters-china-syndrome-meltdown-is-destroying-cement/#more-13499

also:
Dec 1 – Rumor spreading in Japan says leukemia up 7 times from last year — Champion wrestler who trained in Fukushima diagnosed with acute myelocytic leukemia
Nov 27 – Journalist dies of acute lymphocytic leukemia — Was living inside 30 km zone to support Fukushima
Nov 22 – Japan TV star suffering acute leukemia after eating Fukushima produce (VIDEO)
Nov 7 – Japan TV host diagnosed with acute lymphocytic leukemia — Had been eating Fukushima produce on show
Nov 3 – Hospital worker leaks information about recent increase in acute leukemia
Aug 30 – 40-year-old Fukushima radiation worker dies of acute leukemia after working at plant for week — Checkup showed no prior health problems