Floating Forever in the Depths

Many Victorians, including some scientists, believed that the density of sea water increased with depth. (Actually it does not, or only a little.) They therefore believed that things would sink into the sea only to the depth at which their density matched that of the surrounding water. So all objects with the same density -- say, wooden sailing ships, or human bodies -- would sink to about the same depth and hover there. They also believed that life was impossible in the crushing pressure of the deep ocean. So they imagined that things sinking into the depths would float forever in the dark, and that some parts of the ocean would be so crowded with flotsam, especially sunken ships, that this would be a real obstacle to (for example) laying undersea telegraph cables.

I learned this from Simon Winchester's Atlantic, which is a disorganized and at times tedious book but does include the occasional gem.