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What is it?

Water-collecting cylinders called Niskin bottles stand ready, spring-loaded caps open at both ends. They are most often attached to a CTD rosette sampler–a frame holding a circle of bottles around a central core of instruments that measure Conductivity (saltiness), Temperature, and Depth. Oceanographers lower CTDs at multiple locations along a ship’s cruise track to measure those properties at various depths along their course. At the same time, they signal the CTD to close bottles at selected depths, trapping water from those depths for later chemical or biological analysis.(Photo by Ellen Roosen, Woods Hole Oceanographic Institution)