Now you probably can't wait to learn ALL about this arcane, but exciting technology. First let's see if we can figure out what it does by its name.
Submarine-Launched. Pretty self-explanatory there.
Expendable. OK: you won't be getting it back after launching it into the ocean.
Bathythermograph. It records the temperature of the ocean.
So we have a device that takes the temperature of the ocean, apparently... couldn't we just stick a thermometer into the water to figure that out?
Actually an SSXBT does a little bit more than take water temperature in a single place, which is what makes them pretty cool devices :)
The purpose of an SSXBT is to make a graph of the ocean temperature from the surface, all the way down to the maximum operating depth of the submarine that launched it. The reason for wanting to know the ocean's temperature vs. depth profile is to better understand how sound will carry at various ocean depths, and set your ship's operating depth accordingly.
Colder ocean water is more dense and tends to carry sound further. Interfaces between warmer and colder layers can reflect sound, keeping surface ships from hearing a submarine. A cold channel between two warm layers may carry a submarine's sound a great deal further horizontally to another submarine than it would ordinarily go. Understanding these conditions and using them tactically helps a submarine to remain hidden.
This cannot be emphasized enough: Remaining undetected is critical for a submarine. The only advantage (although it's a huge one) that a submarine has is stealth. Once a submarine is detected, it is no more difficult to destroy than a surface ship. Probably easier, because as floating targets, surface ships have many, many countermeasures against attack, while submarines do not.
...Which brings us back to our SSXBT and getting a profile of the ocean's temperature vs. depth.
Below is an image of an SSXBT, which is 3 inches in diameter and about 3 ft. long. It is shaped like a small torpedo, and in fact, these are gently launched with pumped water through tiny 3" torpedo tubes (called "Signal Ejectors") out the top of the submarine. A signal wire connects the SSXBT to a recorder on the ship (which of course has to be moving very slowly for this to work at all)
In the photo you will notice the SSXBT is made of two pieces. The piece to the right is buoyant. After launch, the SSXBT floats to the surface, and then the right (buoyant) part separates. The left part contains a weighted temperature probe with fins, which records water pressure (depth) and temperature as it sinks. The data is relayed back to the ship via a signal wire, which is wound on a spool in the slotted section of the SSXBT. The buoyant part of the SSXBT scuttles itself after reaching the surface, so there is only brief evidence at the surface that a submarine lurks below.
Below is a drawing of an SSXBT, slightly more complicated than the one above. This one has a "Lifting Body" (#306) that is buoyant, and helps prevent the signal wire from getting tangled up with the ship.
Just so you don't think I am blathering highly classified information all over the internet:
Here is a Lockheed-Martin advertisement with a drawing explaining how their SSXBT works.
Here is a declassified document (10-20 second .PDF download) explaining all about temperature gradients and how to read and troubleshoot your own SSXBT temperature chart! Pretty cool stuff.
The photo of the SSXBT above was from someone who was selling it on Ebay. Unfortunately the auction has already ended. Truly a Christmas gift for the guy who has everything... I'd be willing to bet he doesn't have one of these :)
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