The Valdivia Expedition, led by German marine biologist Carl Chun in 1898-1899, was the first time humans had explored the ocean depths below 500 fathoms. What they found changed our conception of the oceans. The results, in the form of 24 volumes of text and illustrations, took decades to be published. Among the volumes was The Cephalopoda, published in 1910 and filled with colorful hand-illustrated drawings of octopuses and squid, courtesy of the Biodiversity Heritage Library.
I found this on Brain Pickings, which identifies the illustrator as Friedrich Wilhelm Winter, a credit I couldn't find in the actual book itself. They're also selling some of the illustrations as prints, like this one of the octopus featured above.