operating a sub is inherently dangerous. Accidents that reslt in loss of the vessel happen much more frequently than for surface ships. There is just too little room for error or malfunction with a sub.

Yes, a number of U-boats were lost in accidents, varying from collision with another U-boat or surface ship during training to one that was lost when the toilet malfunctioned (seawater flooded in thru the toilet, flooding the battery, which made chlorine gas, forcing the sub to surface near allied ships - now that\'s a story to tell in POW camp....). These accidents didn\'t seem to be unduly common compared to other powers. Don\'t know if they got more common later in the war (in proportion to fleet size).

A number of submarines from every power simply dissapeared without a trace. Did they hit a mine? Friendly fire? An unrecorded enemy attack? Or was it a mistake or mechanical failure? A mix of all, no doubt.