What a surprise.
Workers braved the high radiation (up to 100 millisievert/hour) in the Reactor 3 reactor building on June 9 and found water in the basement. TEPCO says it expected to find water there, but didn't bother to tell us until the water was actually found.
Have I seen this before? Yes, the Reactor 1.
To state the obvious, if there's water in the basement of the reactor building, that means 1) the Reactor Pressure Vessel (RPV) is broken; 2) the Containment Vessel is broken.
The only question is: Where's the corium?
From Asahi Shinbun (10:45PM JST 6/10/2011):
TEPCO announced on June 10 that they confirmed the presence of water in the basement of the Reactor 3 reactor building at Fukushima I Nuclear Power Plant. TEPCO had estimated that 6,400 tons of water would be in the basement as of the end of May, based on the amount of water injected into the RPV. Now its existence has been confirmed.
According to TEPCO, workers entered the reactor building and descended the stairs from the 1st floor to the basement, and saw the water. Depth of the water was 5.8 meters [19 feet] from the basement floor, and the amount of water looked to be slightly more than TEPCO's estimate. The radiation level near the surface of the water was 51 millisieverts/hour.
According to TEPCO, it is possible that the RPV has been damaged by the core meltdown, leaking the water. TEPCO plans to test the water for radioactive materials.
Survey of the 1st floor found distorted doors of the instrument panel and a ladder dropped from the floor above. Part of the equipment was blackened with what looked like soot. TEPCO thinks they were caused by the hydrogen explosion.