French and British nuclear submarines collided in the Atlantic. Such collisions are rare, but do happen from time to time. Both nuclear powered subs were carrying nuclear weapons. The accident happened about a week and a half ago.
Light does not penetrate into the deep water so subs use Sonar or sound to hear. At the same time the sub is listening for potential adversaries, it does everything it can to remain silent as to remain undetected by those adversaries. The effect can be likened to blind men tip towing around a large field trying to hear the others while remaining silent themselves. If both succeed at remaining quite, collisions can result. Despite the vastness of the ocean, the same thing can and does happen.
More at http://www.thesun.co.uk/sol/homepage/news/article2240543.ece.