What accident happened during NASA's Apollo 13 mission?

Apollo 13 was NASA's third moon landing mission, but the astronauts never made it to the lunar surface. During the mission's dramatic series of events, an oxygen tank explosion almost 56 hours into the flight forced the crew to abandon all thoughts of reaching the moon. The spacecraft was damaged, but the crew was able to seek cramped shelter in the lunar module for the trip back to Earth, before returning to the command module for an uncomfortable splashdown. The mission stands today as an example of the dangers of space travel and of NASA's innovative minds working together to save lives on the fly.

Source: Space