The inventor of the skipping bombs used in the dam busters raid, Barnes Wallis, also invented the first bunker busting bombs, designed to take out heavily fortified targets like the U-boat pens on the Bay of Biscay.
http://www.bismarck-class.dk/tirpitz/miscellaneous/tallboy/tallboy.html
Read the book and watched the movie years ago. As I recall, the basic idea of getting the altitude right was discovered when the guy was at a play and spied two spotlights on opposing sides of the theater trained on the lead actor. Viola!