Answer:
In the most severe reactor accident, the fuel will melt and, due to radioactive decay heat, will continue to be very hot. In fact, it will be hot enough to melt through the bottom of the reactor pressure vessel (several inches of steel), and possibly melt/burn through the concrete floor of the reactor building and get into the soil beneath the building. This is what is referred to as the China syndrome, the idea being that the molten mass of fuel is heading toward China on the other side of the earth as it melts through the vessel, concrete, and then soil and rock below the reactor building.