Underwater Depth (also known less interestingly as 'depth contours' - bath is old Greek for deep, from which we also have the word bathos, a literary term describing when serious elevated speech or writing becomes trivially comic or daft - not to be confused with the word bath, as in wash and bathe, which is derived from Old High German, notably bad, meaning to warm, from which similar roots evolved the word bake)   _{Your Comment}