CTD – which is short for Conductivity-Temperature-Depth – is indeed a much appreciated instrument by every (sea-going) oceanographer. You send it down to the bottom of the ocean, and back comes nice profiles of temperature, conductivity (from which one can calculate salinty) – and whatever other sensor you’ve attached (oxygen, chlorophyll, and turbidity for example)
Most of the time the CTD is mounted on a rosette, which carries bottles so that one can also collect water samples from selected depths. But make sure to have the bottles open when you send them down – otherwise they will implode, and that is not a good thing, believe me!
