OBS observation of underwater explosion and natural earthquakes contributes in many respects to the ocean acoustics for understanding the nature of underwater sound propagation and acoustic properties of bottom sediments as well as subbottom structure. It was observed thatT‐phases were generated in a deep flat ocean basin by the earthquakes which occurred in the ocean crust. It is interpreted that the abyssalT‐phase is generated by the diffraction wave at the oceanic basement (layer II) which is overlying very close to the shallow earthquake origin. The arrival phases of underwater sound from explosion source to the OBS in the long range propagation in a deep ocean generally contain very small onset phases. This is regarded as the limiting ray of RSR (Refraction Surface Reflected) rays. An advantage of using OBS is to obtainS‐wave information.P‐wave toS‐wave conversion takes place very efficiently at the sea bottom as well as at the base of sediment layer.S‐wave velocity in the bottom sediment (layer I) is very low, the ratioVp/Vp ≑ 4 ∼ 3. The spectra of seismic noise upon the deep ocean floor decrease very rapidly above several Hz.