Bruce H. Robison, a marine biologist at the Monterey Bay Aquarium Research Institute in California, began prowling the deep Pacific in a revolutionary craft in 1985. It was essentially a giant bubble of clear plastic that gave its occupant stunning panoramic views, instead of requiring them to peer through a tiny porthole.

“It was absolutely transformative,” Dr. Robison said recently . “The profusion of life was so much greater than what I had imagined.” The dark sea was alive: glowing, flashing, shimmering. “It was amazing to see all this bioluminescence and realize it’s a major form of communication,” he said. “It really changes your perspective.”

Three-plus decades later, bubble craft have gone mainstream, and thousands of people are experiencing that deep-sea vista. While Elon Musk and Jeff Bezos advance space travel, another set of entrepreneurs is going in the opposite direction, seeking to expand the exploration of inner space. Fans of the undersea craft sometimes call these new submersibles inner spaceships.

“They keep reaching deeper and deeper,” said Will Kohnen, who tracks development of bubble craft for the Marine Technology Society, a professional group. Much of the activity, he added, arises from growing concern about the ocean’s health: “People want to see it firsthand. It’s all about connecting with the ocean.”