Chernobyl disaster
Ukrainian scientists report that, for an unknown reason, the levels of nuclear radiation have increased in the remains of the Chernobyl power plant. Most areas of containment have shown decreasing radiation levels, however, in one particular room, radiation counts have doubled over the last four years. These radiation levels are high enough to preclude installing sensors. Additionally, fuel containing materials, which were initially the consistency of lava, are disintegrating into radioactive dust.
Sources: Nature Popular Mechanics