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The Inverted Echo Sounder (IES) is an ocean-bottom moored instrument which accurately measures the time required for an acoustic pulse to travel from the sea floor to the ocean surface and back. The round-trip acoustic travel time varies in response to changes in the mean temperature profile of the water column above the instrument, as well as changes in water depth. The instrument is used as a sensitive indicator of changes in the main thermocline depth caused by synoptic-scale eddies or the shifting path of an ocean current. The IES is small (17"-diameter glass sphere) and self-contained, with its own acoustic release, relocation and recovery system. Its battery capacity and digital tape recorder (data capacity 107 bits) allow it to be deployed for up to 18 months. It can operate in water depths of up to 6700 m and requires only an anchor as mooring equipment. Microprocessor-based electronics allow programmable data formatting and sampling cycles. Additional data channels for pressure, temperature, and ambient noise are optional. The system has undergone extensive development and field testing, resulting in a reliable, cost-effective means to study temporal variability in large-scale features of the temperature field of the oceans.