I have heard reports that a hole has been found in the lower part of the steel can which holds the nuclear reactor in unit one at Fukushima. This steel can is known as the pressure vessel.
This hole means that the accident is worse than it was originally thought to be, if you look at the following two summaries of the status of important parts of the plant you will see near the top of the column for unit one the big difference.
While this is not good it is important to understand that the heat generation in the nuclear fuel is now much less than it was on the day of the nasty big earthquake. As time goes on the heat production will become less and less, so you do not have to worry about a permanently molten pool of fuel.
It is important to note that while molten fuel can damage concrete, it will be unlikely to melt its way deep into the earth because as the fuel melts its way through objects such as the pressure vessel and the cladding that the fuel will be mixed with more and more inert non radioactive matter. This will dilute the radioactive lava, by diluting the radioactive lave and spreading it out on the floor of the containment the temperature of the molten fuel will go down. It will be likely to form a solid lump like the lava flows in the basement of Chernobyl did.
The fact that the accident in unit one progressed to this stage will make the clean up operation harder, most costly and longer to perform.