Three British submarine officers, including the captain, pled guilty at their recent court martial. The three were accused of not doing their jobs properly, and allowing their boat, the HMS Superb, to collide, on May 26, 2006, with underwater rocks, that were clearly marked on charts. The sub suffered damage to the bow and sonar, and was not repaired, because the boat was scheduled to be decommissioned shortly anyway, after 32 years of service.
The Superb collision was one of many recently, that have occurred because the crew was not paying enough attention. Last year, for example, a Chinese sub collided with a sonar array being towed behind a U.S. destroyer. Around the same time, a U.S. sub collided with an American amphibious ship in the Persian Gulf (where American subs have suffered two other such incidents in the last five years). Two years ago, an Indian sub, while surfacing near Mumbai, collided with a merchant ship. Five years ago, an American sub, travelling at high speed in the Pacific, collided with an underwater seamount. In one of the most unusual incidents, about a year ago, British and French SSBNs (missile carrying subs) collided with each other while submerged in the Atlantic.
Subs underwater are running blind, as most depend on passive sensors most of the time. Constant attention must be paid to charts and electronic location devices. Crews are intensively trained to stay sharp and be careful when travelling submerged. But it's difficult to keep everyone sharp all the time, and that's what leads to many of these collisions.
The Golden Age of submarine collisions was during the Cold War (1948-91). Once Russia began building nuclear subs in the 1960s, and putting them to sea often and for long periods, there were lots of collisions. Well, about one every two years. Most involved at least one Russian boat. The problem was that the Russians had pretty poor sonar, so they were the equivalent of half blind under water. From the 1970s on, the U.S. has increasingly superior sonar compared to the Russians. This led to the more collisions involving Russian and U.S. boats. It also saw the invention, by the Russians, of the "Crazy Ivan" maneuver. This occurred when an American sub was stalking a Russian one (often an American SSN keeping tabs on a Russian SSBN). The U.S. boat would stay in the Russian subs "blind spot" (behind its propeller). But sometimes the Russians would suspect they were being stalked, or just wanted to make sure they were not, and would perform the "Crazy Ivan" maneuver, which involved upping speed and making a sharp turn. The U.S. sub would have to quickly get out of the way, or there would be, and sometimes was, a collision.
Most of the collisions during this period involved Russian subs bumping into other Russian subs, or inanimate objects (icebergs, oil rigs). Western boats had far fewer collisions because they had better sonar, and better trained and more experienced crews. There are few Russians subs at sea these days, so most of the collisions are by Western subs, which now dominate the ocean depths.