RECENT DEVELOPMENTS in underwater acoustic communication enable real-time contact with underwater sensors, vehicles, and other instruments, offering novel ocean observational capabilities that may change future oceanographic operations. This article summarizes the current state of the art in underwater acoustic communication and telemetry equipment. Acoustic transmission is an attractive option for underwater communication only because radio frequency (RF) and optical energy are severely attenuated under water, whereas acoustic energy propagates well. Information encoded and transmitted on a modulated acoustic waveform can be received and decoded by distant underwater receivers.The relatively slow speed of sound (compared with the speed of light) causes extensive multipath fluctuation and Doppler shifts of the received waveforms. The relative complexity of the oceanic waveguide causes strong spatial, temporal, and frequency-dependent fluctuation. These effects are much more pronounced than those in RF propagation and require rather different approaches to system design. Historically, straightforward "grafts" of RF communication methods to the ocean environment have met with poor success.Underwater communication started with the underwater telephone or "'Gertrude" shortly after WWII. Its derivatives are in common use today, primarily for voice communication between divers and with submersibles. In the 1960s there were several attempts at digital communication (Baggeroer, 1984). By and large, communication technology of the time was inadequate for a waveguide as complex as the underwater channel. In the 1970s and 1980s digital microprocessor tcchnology became available and turned out to be the key enabling technology for underwater modems. Several

We describe the scientific purposes and experimental set-up of an international deployment of a 3 component broadband seismometer package on the ocean floor in Monterey Bay which took place during the summer of 1997. Highlights of this experiment were the installation, performed using a remotely operated vehicle (ROV), the underwater connection of the different components of the package, and the successful retrieval of 3 months of broadband seismic and auxiliary data. Examples of recordings of teleseisms and regional earthquakes are presented and the background noise characteristics are discussed, in comparison with those of near-by broadband land sites, current-meter data from the vicinity of the ocean bottom package, as well as pressure data from deeper ocean sites.

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