An American Navy destroyer collided with a merchant container ship off the coast of Japan early Saturday morning. The USS Fitzgerald which is under its own power with limited propulsion, has taken on water but isn't in danger of sinking. The Navy has confirmed one sailor was injured, but reports indicate seven American sailors are missing.

The Navy is working with the Japanese coast guard to respond to that injury, it said in a statement, and is determining the state of other personnel. It has dispatched two tugs to the scene, and is readying aircraft to respond as needed. A Navy spokesperson could not comment on the state of the container ship's crew.

Once the danger has passed, the investigation into how this happened will begin. That will likely take months, but collisions like this can usually be traced to a few culprits, says Tom Dyer, a maritime consultant and Navy veteran.

The Fitzgerald , a 505-foot-long Arleigh Burke-class guided missile destroyer, likely carried collision avoidance electronics, Dyer says. There's no indication what sort of equipment the ACX Crystal , a Philippine-flagged 732-foot container ship, carried, but even less sophisticated radar systems can offer software that calculates trajectories and alerts operators when they're on a collision course.

Barring a mechanical error?in these systems or, say, one ship's steering capacity?the collision likely resulted from a mistake, or a series of mistakes. "It's usually human error," Dyer says. Ships follow international "rules of the road" when they're in the same area, which specify how what actions to take in myriad situations, but there's no air traffic controller-equivalent giving instructions to keep everyone separated.

Most likely, the crews on one or both ships just did the wrong thing or miscalculated. "People don't always remember exactly what they're supposed to do," Dyer says. Because these big ships are hardly nimble, mistakes can prove hard to correct. "Maneuvering vessels of this size is not easy, and they don't respond quickly if someone makes a mistake."

It's not so different from walking toward someone on the sidewalk where you both move left, then both move right?just in very slow motion, and with a louder crunch when you bump together.

Ships collide more often than you might think. Dyer estimates at least one minor mishap happens somewhere on the ocean every day, and you'll see more serious accidents several times a year. The only thing to do is to study what happened?and try to remember the lessons for next time.