Here there be [the original] monsters.

We've talked a lot about how, from the late 18th through the early 20th centuries, science and technology really did follow the ever accelerating exponential curve people like to impose upon it, culminating with a period of plosive change stretching roughly from 1880 to 1910. Oceanography provides a great example.

Efforts to study the oceans in a systematic and scientific way date back to the late 18th century with researchers like Ben Franklin taking extensive measurements of wind, currents, air and water temperature, and other variables. Below a couple of hundred fathoms, however, the seas were a complete mystery.

For a while, the prevailing theory was that, since water temperature steadily dropped as you went down the first few fathoms, the deeper parts of the ocean were filled with ice. When evidence started contradicting this hypothesis, it was replaced with the idea of a lifeless zone extending most of the way down. This theory was contradicted when engineers repairing transatlantic telegraphs dredged up cables from the ocean floor often pulling inhabitants up in the process.

As with so many other stories of science and technology, modern oceanography basically began in the last quarter of the 19th century with the voyage of the Challenger.

From Wikipedia:

The Challenger expedition of 1872–76 was a scientific exercise that made many discoveries to lay the foundation of oceanography. The expedition was named after the mother vessel, HMS Challenger.

Prompted by Charles Wyville Thomson—of the University of Edinburgh and Merchiston Castle School—the Royal Society of London obtained the use of Challenger from the Royal Navy and in 1872 modified the ship for scientific tasks, equipping her with separate laboratories for natural history and chemistry. The expedition, led by Captain George Nares, sailed from Portsmouth, England, on 21 December 1872.[1] Other naval officers included Commander John Maclear.[2] Under the scientific supervision of Thomson himself, she traveled nearly 70,000 nautical miles (130,000 km) surveying and exploring. The result was the Report Of The Scientific Results of the Exploring Voyage of H.M.S. Challenger during the years 1873-76 which, among many other discoveries, cataloged over 4,000 previously unknown species. John Murray, who supervised the publication, described the report as "the greatest advance in the knowledge of our planet since the celebrated discoveries of the fifteenth and sixteenth centuries". Challenger sailed close to Antarctica, but not within sight of it.
...

Findings from the Challenger expedition continued to be published until 1895, 19 years after the completion of its journey. The report contained 50 volumes and was over 29,500 pages in length. Specimens brought back by Challenger were distributed to the world's foremost experts for examination, which greatly increased the expenses and time required to finalize the report. The report and specimens are currently held at the British Natural History Museum and the report has been made available online. Some specimens, many of which were the first discovered of their kind, are still examined by scientists today.

.This was followed by a number of other expeditions, the most notable for our purposes being...

The Valdivia Expedition, or Deutschen Tiefsee-Expedition (German Deep Sea Expedition), was a scientific expedition organised and funded by the German Empire under Kaiser Wilhelm II and was named after the ship which was bought and outfitted for the expedition, the SS Valdivia. It was led by the marine biologist Carl Chun and the expedition ran from 1898-1899 with the purpose of exploring the depths of the oceans below 500 fathoms, which had not been explored by the earlier Challenger Expedition. 

This is yet another example of our understanding of the world shifting from primitive to relatively modern in the late 19th/early 20th century. If you could have a conversation with a group of well educated layman in the 1870s, you would find that they were almost completely ignorant about the nature of life near the bottom of the ocean. If you gathered a similar group 25 or 30 years later, you would discover a remarkable amount of common ground between their ideas and yours. It's entirely possible, that some of the pictures of the strange creatures you have swimming around in your head are actually derived from the samples dredged up by the Valdivia Expedition.

.If you'd like to get a sense of how people in the early 20th century viewed these discoveries, I would recommend Willy Ley's essay for Galaxy Magazine where he describes childhood visits to the museum that housed the preserved samples from the voyage. The essay is also the source of the illustrations for this post.

Finally, there's one more point I'd like to make about the way our view of the world (and beyond) changed radically in this period. It is become common, perhaps even standard, to describe these creatures as "alien," or looking like they "came from another planet." I suspect this gets the relationship exactly backwards. It's not that these strange animals remind us of what we imagine extraterrestrial life to be; it's that we now tend to imagine extraterrestrial life has something like you find at the bottom of the ocean.