How interesting. Thanks for the link Ethel and thanks for your view tiggerblue, which could be the case. Ah well, I won't lose sleep over that tonight then.
Google have already explained it's a side effect of their software. The ocean floor is mapped using sonar or something similar and then it's portrayed using software, extrapolating where necessary.
Too late Francis Asis, I didn't know my next door neighbour already had one (the whole thing) and he's going to let me borrow it for the week-end. It also has a sat-nav so I shouldn't get lost. Not too sure if the wife wants to come with me. If she does, it'll be the same old thing.