Some people believe that the sound you hear when putting a seashell to your ear is that of your blood rushing though the blood vessels in the ear, but that is not true. It is the ambient noise that's produced when the air entering the seashell, The unique shape of seashells amplifies the ambient sound, which means that any air that makes its way through the seashell produces sound when bounced about in the curved inner surface. The sound that is produced sounds ocean-like but isn't.