How deep a body of water is can be measured from the surface of water to the bottom. You should always be sure you know how deep a swimming pool is before demonstrating your backward somersault dive.
Water can be deep or shallow, and exactly how deep it is can be measured. You can also describe a space not made of water as deep — like when you discover a little house made of candy deep, or far, in the woods. Things can even be figuratively deep when they are extreme, like "deep thoughts" or a "deep recession." The Old English word deop means "deep," but it also means, "awful, mysterious, and solemn."