Some writers go to the ends of the earth in pursuit of a great story. Susan Casey went to the bottom of the ocean.

While researching her new book, “The Underworld,” about the otherworldly inhabitants of the deep ocean and the explorers and scientists who are surveying these uncharted depths, Casey ventured down in deep sea submersibles, visiting eerie, alien landscapes that no humans had ever seen.

The deep sea makes up more than 90 percent of earth’s biosphere, yet stunningly little is known about it. That’s starting to change, with new technology, like smart drones that are mapping the ocean floor, and with expeditions in cutting-edge submersibles by private explorers like the director James Cameron and the private equity investor Victor Vescovo, who