The rectangular buildings housing reactor units 4, 3, 2, and 1 (left to right) show varying levels of damage at Japan's Fukushima Daiichi nuclear plant on Wednesday. In each building, after cooling systems had failed, fuel rods began heating up, spurring explosions in spaces between the reactor itself and the building's exterior walls.
Unit 4's roof (far left) appears intact, but much of the building's sides were destroyed March 15, when chemical reactions with spent fuel rods likely sparked hydrogen-gas explosions and fires, according to the New York Times.
At Unit 3, pictured spewing white steam, a March 14 blast wrecked the building and harmed the steel containment vessel around the nuclear reactor itself.
At Unit 2—appearing the least damaged but with an exterior panel likely removed to vent steam, according to the Institute for Science and International Security—a March 15 explosion likely compromised the reactor's primary containment shell.
At Unit 1 (far right) a March 12 blast destroyed roughly the top third of the building.