After nearly three months of hemming and hawing, the government of Japan now admits that in each of the three melted-down reactors at Fukushima, the nasty radioactive lava known as corium has broken through the reactor pressure vessel and flowed down into the containment vessel that surrounds the reactor core. The reactor vessel has a shell of six to eight inches of steel, but it's been weakened at Fukushima by the combined effects of exposure to sea water and extreme heat. Besides, these reactors are old -- beyond their design lives -- and there are a number of vulnerable spots where pipes and tubes get connected.

With the corium out of the reactor vessel, the containment vessel is the last hope to keep the melted fuel within the plant. It's basically a thick concrete pad all around and beneath the reactor; the earth and the groundwater are immediately below. The classic drawing is here; the containment vessel is that yellow part.

The groundwater beneath the plant is already being contaminated from radioactive runoff from the desperate efforts to cool the mess. But if the lava itself falls into the water table, along with bubbled-off concrete, then a large, shallow underground steam explosion could occur. Given that the plant is right next to the ocean, there isn't all that much earth between the surface and the groundwater.

In other news that reveals how entirely bogus the pronouncements from the government and plant operator Tokyo Electric have been, yesterday they changed their estimates of how much radiation was released from Fukushima in the early days following the meltdowns:

According to news reports, NISA [the Japanese nuclear regulatory agency] now estimates the total amount of radiation released into the atmosphere in the first week of the crisis at 770,000 terabecquerels. This compares with NISA's previous estimate, released on April 12, of 370,000 terabecquerels for the first month of the crisis. NISA has pointed out that most of the radiation was released in the first week.

And that's just the airborne stuff -- what we've been breathing the last few months. It says nothing of what's been dumped into the western Pacific Ocean, where a lot of our seafood swims.

Meanwhile, here's a YouTube video from the outskirts of Tokyo that's sure to be going viral. It appears to have been shot within the last day, 135 miles south of Fukushima. Although the Geiger counter is pretty mellow when it's above ground level, watch what happens when the holder places it down near the sewer grate. The reading jumps to more than 5.5 microsieverts per hour. Even the recently (and obscenely) relaxed Japanese bureaucrat standard for playgrounds much closer to the meltdowns is 3.8 microsieverts per hour. At that level or above, the kids aren't allowed outside.

Obviously, there's been a fair amount of radiation in the air over Tokyo, and the rain has been washing it onto the ground and into the sewer system. It's not exactly safe there. Heaven help the people of Japan, and everyone around the world who's in the shadow of one of these ultra-dangerous facilities.