where it is. As the mass oscillates through the magnetic field it produces an electric current which the instrument measures. The seismometer itself is a small metal cylinder; the rest of the footlocker-sized OBS consists of equipment to run the seismometer (a data logger and batteries), weight to sink it to the sea floor, a remote-controlled acoustic release and flotation to bring the instrument back to the surface. Types of OBS The ground motion caused by earthquakes can be extremely small (less than a millimeter) or large (several meters). Small motions have high frequencies, so monitoring them requires measuring movement many times per second and produces huge amounts of data. Large motions are much rarer, so instruments need to record data less frequently, to save memory space and battery power for longer deployments. Because of this variability, engineers have designed two basic kinds of seismometers: Short-period OBSs They record high-frequency motions (up to hundreds of times per second). They can record small, short-period earthquakes and are also useful for studying the outer tens of kilometers of the seafloor. Technical details for two models: WHOI D2 and Scripps L-CHEAPO. Long-period OBSs They record a much broader range of motions, with frequencies of about 10 per second to once or twice a minute. They are used for recording mid-sized earthquakes and seismic activity far from the instrument. Technical details for two models: WHOI long-deployment OBS and Scripps long-deployment OBS. Custom OBSs Custom OBS are beginning to be developed, as the need for expansion on coverage in the field of seismology increases and permanent deployments are necessary. One customizations to improve the data quality of the seismometers is to borehole the seismometer in an aluminum casing into the surface (~1 m) to create stability in the soft sediment of the ocean floor. Another customization that is possible is to add a differential pressure gauge (DPG) and/or current meter, in order to understand how the pressure is changing around the seismometer. It can also be practical to store the datalogger and battery in a glass Benthos sphere in order to be able to connect to the ship through the use of a remotely operated vehicle (ROV), which is a necessary advancement in order to have and maintain permanent OBS deployments. Advantages Very stable clocks make comparable the readings from many far-flung seismometers. (Without reliable time-stamps, data from different machines would be unusable.) Development of