A stable platform from which to perform experiments at sea has been a long-sought goal of sea-going scientists. In order to perform fine-scale experiments on fluctuations of acoustic signals in the ocean the manned buoy FLIP has been built to provide the needed stable platform for such work. The 355-ft-long, 600-ton craft is towed to station in the horizontal position, and operates in the vertical position at a draft of 300 ft with its electronics laboratory about 30 ft above the water line. A description is given of its development, stability, and operating characteristics, including the unique flipping operation.