2.1 Mapping the deep
Until the late 1930s the only way to measure the depth of the ocean was to use a line with a weight on the end. For example, in 1521 Ferdinand Magellan stopped his ships in the Pacific Ocean during the first circumnavigation of the globe and lowered such a line. After paying out all the line they had he was sure that the weight had not touched the bottom, but how deep was it? He knew the ocean was deeper than his 400 fathoms of rope (~730 m) - so he concluded that his ship was over the greatest depths in the oceans!