Which sea was considered to be non-navigable?

The Atlantic Ocean's Sargasso Sea was named after the Sargassum seaweed, which floats en masse on the surface there. Although it was formerly thought to cover the entirety of the Sargasso Sea, making navigation impossible, it has since been found to occur only in drifts. The historic incidents of sailing ships being trapped there are now considered due to the often calm winds of the horse latitudes.