How were water depths and seafloor bathymetry (submarine topography)determined by the Challenger surveys?

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The Challenger surveys, conducted in the late 19th century, primarily used hand-thrown and retrieved weighted lead-lines to measure water depths and map the seafloor. This method involved lowering a lead weight attached to a line into the water until it reached the seabed, allowing for the measurement of depth.

How Were Water Depths and Seafloor Bathymetry (Submarine Topography)determined by the Challenger