Two nuclear subs ram into each other

Look at the news below:

http://news.yahoo.com/s/afp/20090217/wl_afp/britainfrancenuclearweapons

When I read it, I thought it was funny. They say a couple of nuclear (unclear) subs rammed into each other and did not know that they rammed into each other. Well, it is like two old people kissing each other and not knowing what they were doing. Why should we have such weapons that cannot be detected by one another? I am surprised to imagine that they were not even equipped with a plain periscope or some such thing.

Or is it the case of the more invisible thing winning?