I was reading about depth sounding and the various devices that were used, but it doesn't really explain the practicalities of how 19th century depth sounding worked. With the Challenger Deep, for instance, the crew of the Challenger estimated the depth at roughly 27 000 feet. How did the ship have that much rope lying around? How did the crew know when the weight on the rope hit the bottom and didn't just snag on something? How did the rope get hauled back in? How did ships like the Challenger choose where to sound the depths?
The Challenger expedition had on board a Baillie sounding machine equipped with hemp No.1 line weighing 200 pounds to the nautical mile. The machine had a mechanism to monitor the line tension which they would watch to detect when the weight hit bottom. It wasn't perfect but with experience and sufficient weight on the end they could reliably detect the bottom impact. A bigger issue was slanting of the line due to ocean currents which would push the weight sideways resulting in a overestimate of the depth. Outside of regions of strong currents (such as the Gulf Stream) wire angle was usually not too big of a problem. There is very little in the ocean for the line to snag on to cause a underestimate of the depth. To haul the line back, the Challenger was equipped with steam-powered winches.