Over a wide range of wavelengths the ocean is opaque to electromagnetic radiation but relatively transparent to sound. This property motivates a growing band of oceanographers to explore the possibilities of underwater acoustics as a tool for ocean measurement. The applications span a wide range. At the largest scales, the temperature dependence of sound speed allows monitoring of path-averaged temperature variability over ocean basin scales with great precision. This is the principle on which acoustic thermometry of ocean climate is based, a remarkable story in its own right with political as well as scientific challenges.