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Five miles of cable

A CTD, which  stands for Conductivity, Temperature, and Depth, is one of the most basic tools for understanding the ocean. The instrument is made up of a set of small probes attached to a large metal rosette wheel and is lowered on a cable down to the seafloor. The CTD’s cable is special — it holds the instrument and it also carries data. The CTD sends measurements back to the ship in real time, and the operator on the ship sends back signals for it to close its bottles. The cable is 8,000 meters long, so the CTD can be used in very deep water. That means the data has to travel five miles even if the CTD is just below the ship.
(Photo by Chris Linder, Woods Hole Oceanographic Institution)