Chernobyl disaster
Ukrainian scientists report that, for an unknown reason, nuclear reactions have increased in the remains of the Chernobyl power plant. Most areas of containment have shown decreasing radiation, yet an inaccessible room 305-2 radiation counts have doubled over the last 4 years. Radiation levels are high enough to preclude installing sensors. Further complicating matters, fuel containing materials, initially the consistency of lava, are disintegrating into radioactive dust.
Sources: Nature Popular Mechanics