Japan’s nuclear crisis is intensifying. A second reactor unit at the damaged Fukushima Daiichi nuclear power plant may have ruptured and appears to be releasing radioactive steam. It is unclear how serious the breach might be, but the vessel that possibly ruptured is the last fully intact line of defense against large-scale releases of radioactive material. Several explosions have hit the plant since Friday’s devastating earthquake. The radiation levels around the plant are so high that Japanese authorities abandoned a plan to dump water from military helicopters in an attempt to cool the reactors. More workers are expected to return to the plant today after radiation levels forced their evacuation.