A device developed by Athelstan Spilhaus in 1938 to measure temperature/depth profiles in the ocean, the bathythermograph was basically a reworking of a mostly unworkable device called an oceanograph built in 1934 by Carl-Gustav Rossby for the same purpose. It consisted of an open, rectangular frame in which a compressible bellows with a pen arm and stylus was mounted at one end. The stylus rested on a smoked glass slide and moved across it to scratch a record of ocean temperatures. The stylus also moved vertically with changes in depth and thus created a temperature/depth profile.

The bathythermograph (or BT) was further improved by Maurice Ewing and Allyn Vine in 1940. Their version responded more quickly to temperature changes and was streamlined so it could be lowered and raised more quickly from a moving ship than could the previous more unwieldy version. In 1940 WHOI started doing military research for the government, a large part of which was concerned with sonar and the use of BTs with it. Knowledge of the vertical temperature structure of the ocean was extremely helpful to sonar operators since sound speed in sea water is a strong function of temperature, and various types of vertical temperature profiles would lead to sound traveling differently in the ocean. BT data was also useful for adjusting the buoyancy or trim of submarines since it could help provide an estimate of how much ballast would be needed to move a submarine from periscope depth to greater depths. A strong thermocline would require much more ballast for the submarine to descend.

The military research also led to further improvements in the BT including better aerodynamics for more stable operation at higher speeds as well as moving the glass slide and stylus from within the BT to inside the submarine. By early 1943 many submarines were outfitted with and used BTs. In an extremely helpful quid pro quo, the glass slides were given to WHOI and Scripps after missions in both oceans, allowing charts of the vertical temperature structure of the ocean to be constructed. Over 60,000 slides from the North Atlantic alone were thus made available to oceanographers.