Abstract: The stratified structure due to temperature and salinity in-homogeneity inthe ocean is firstly presented in the current paper. With minor restoring force, theinternal wave is characteristic of larger amplitude and lower frequency than thecounterpart of the sea surface wave. Internal waves occur along the belt of continentaledge with suitable stratification, current and topography in various forms of internaltide, internal soliton etc. on this planet. The activities of internal waves appear ashidden trouble for offshore structure safety, background noise in the underwateracoustic channels and a ``stirrer'' for abyssal mixing. We have observed variousapplications in ocean engineering, global environment and underwater acousticexploration.