Inventors in England sent improved deep-sea thermometers to the Challenger for testing. They'd developed the new reversing thermometer to solve the problem Challenger scientists faced: how to measure the water temperature at a determined depth, without assuming that the coldest water was necessarily at the ocean bottom.

The Challenger scientists lowered the new reversing thermometer to the chosen depth. Then, by pulling up on the attched rope to release a catch, they flipped the thermometer upside-down and recorded the temperature at that specific depth.