Japan nuclear Fukushima - how much water do you need to cool it?

Why is it such a big deal to cool these reactors? I understand that the contents of the reactor keep producing "decay" heat for days after they are shut down, but how much water are we talking about? Couldn't they pour it in with a bucket, or if it's more, drive in a tanker truck and deliver it that way? Haven't seen any news report that gives the slightest idea -- to help understand why it is such a big challenge.