I remember this story about a u-boat in the Bay of Biscay. They were travelling submereged, and all of a sudden could hear a scraping sound. The skipper concluded pretty quickly that it must be a mine, or the chain more precisely, and fearing that it could snag onto something and pull the mine to their boat he ordered a turn(don't remember which way). The scraping sound moved towards the rear as tension increased, until at some point.... BOOOOM!
I don't remember the boat, when it happened or where I read this, but try to find out. If it's true then there obviously must have been survivors.
edit: Well, it looks like there are several possible boats. For example
U-526 and
U-171