The bathythermograph, or BT, also known as the Mechanical Bathythermograph, or MBT;^[1] is a device that holds a temperature sensor and a transducer to detect changes in water temperature versus depth down to a depth of approximately 285 meters (935 feet). Lowered by a small winch on the ship into the water, the BT records pressure and temperature changes on a coated glass slide as it is dropped nearly freely through the water.^[2] While the instrument is being dropped, the wire is paid out until it reaches a predetermined depth, then a brake is applied and the BT is drawn back to the surface.^[1] Because the pressure is a function of depth (see Pascal's law), temperature measurements can be correlated with the depth at which they are recorded.^{[citation needed]}

The true origins of the BT began in 1935 when Carl-Gustaf Rossby started experimenting. He then forwarded the development of the BT to his graduate student Athelstan Spilhaus, who then fully developed the BT in 1938^[1] as a collaboration between MIT, Woods Hole Oceanographic Institution (WHOI), and the U.S. Navy.^[3] The device was modified during World War II to gather information on the varying temperature of the ocean for the U.S. Navy. Originally the slides were prepared "by rubbing a bit of skunk oil on with a finger and then wiping off with the soft side of one's hand," followed by smoking the slide over the flame of a Bunsen burner.^[4] Later on the skunk oil was replaced with an evaporated metal film.^[1]

Since water temperature may vary by layer and may affect sonar by producing inaccurate location results, bathothermographs (U.S. World War II spelling) were installed on the outer hulls of U.S. submarines during World War II.^[5]

By monitoring variances, or lack of variances, in underwater temperature or pressure layers, while submerged, the submarine commander could adjust and compensate for temperature layers that could affect sonar accuracy. This was especially important when firing torpedoes at a target based strictly on a sonar fix.^[5]

More importantly, when the submarine was under attack by a surface vessel using sonar, the information from the bathothermograph allowed the submarine commander to seek thermoclines, which are colder layers of water, that would distort the pinging from the surface vessel's sonar, allowing the submarine under attack to "disguise" its actual position and to escape depth charge damage and eventually to escape from the surface vessel.^[5]

Throughout the use of the bathythermograph various technicians, watchstanders, and oceanographers noted how dangerous the deployment and retrieval of the BT was. According to watchstander Edward S. Barr:

"… In any kind of rough weather, this BT position was frequently subject to waves making a clean sweep of the deck. In spite of breaking waves over the side, the operator had to hold his station, because the equipment was already over the side. One couldn't run for shelter as the brake and hoisting power were combined in a single hand lever. To let go of this lever would cause all the wire on the winch to unwind, sending the recording device and all its cable to the ocean bottom forever. It was not at all uncommon, from the protective position of the laboratory door, to look back and see your watchmate at the BT winch completely disappear from sight as a wave would come crashing over the side. … We also took turns taking BT readings. It wasn't fair for only one person to get wet consistently."^[6]

After witnessing firsthand the dangers of deploying and retrieving BTs, James M. Snodgrass began developing the expendable bathythermograph (XBT). Snodgrass' description of the XBT:

Briefly, the unit would break down in two components, as follows: the ship to surface unit, and surface to expendable unit. I have in mind a package which could be jettisoned, either by the "Armstrong" method, or some simple mechanical device, which would at all times be connected to the surface vessel. The wire would be paid out from the surface ship and not from the surface float unit. The surface float would require a minimum of flotation and a small, very simple sea anchor. From this simple platform the expendable BT unit would sink as outlined for the acoustic unit. However, it would unwind as it goes a very fine thread of probably neutrally buoyant conductor terminating at the float unit, thence connected to the wire leading to the ship.^[7]

In the early 1960s the U.S. Navy contracted Sippican Corporation of Marion, Massachusetts to develop the XBT, who became the sole supplier.^[1]

The unit is composed of a probe; a wire link; and a shipboard canister. Inside of the probe is a thermistor which is connected electronically to a chart recorder. The probe falls freely at 20 feet per second and that determines its depth and provides a temperature-depth trace on the recorder. A pair of fine copper wires which pay out from both a spool retained on the ship and one dropped with the instrument, provide a data transfer line to the ship for shipboard recording. Eventually, the wire runs out and breaks, and the XBT sinks to the ocean floor. Since the deployment of an XBT does not require the ship to slow down or otherwise interfere with normal operations, XBT's are often deployed from vessels of opportunity, such as cargo ships or ferries, and also by dedicated research ships conducting underway operations when a CTD cast would require stopping the ship for several hours. Airborne versions (AXBT) are also used; these use radio frequencies to transmit the data to the aircraft during deployment. Today Lockheed Martin Sippican has manufactured over 5 million XBTs.

• Oceanography and hydrography: to obtain information on the temperature structure of the ocean.
  • A study in 2019 (published 2023) at the outfall of the Totten Glacier in East Antarctica showed that water at depth above freezing temperature was melting the under-side of the glacier.^[13][14]
• Submarine and Anti-submarine warfare: to determine the layer depth (thermocline) used by submarines to avoid active sonar search.

• Depth sounding – Measuring the depths of a body of water
• Benthic lander – Platform for measurements on the seabed
• CTD – Device to measure seawater properties