Space flights have never been cheap or easy, but the human species has proved to have the right stuff when it comes to rocketing skyward. Despite the costs and complications, we—at least those of us in the celestial-minded United States—are still talking seriously about sending crews of men and women back to the moon, Mars, an asteroid, maybe even beyond.

But what ever happened to heading in the other direction, into the ocean depths? Half a century ago, it seemed only natural that we would be sending manned missions into the deep sea just as we were launching astronauts into space. Most of the world’s oceans, along with the terrain they cover, were and still are unseen and unexplored, and whatever we discover down there would very likely heighten our understanding of our very own home planet.

In that spirit, at the dawn of the Space Age, the U.S. Navy launched the Sealab program to figure out how to accommodate “aquanauts” on the ocean floor so they could live, work, and explore, like astronauts at a space station or on the moon. Nothing like this had ever been done, and not everyone in the Navy was convinced it was a good idea to try. Yet where matters of exploration are concerned, it counts to have people in the picture, weathering the elements, taking those historic first steps, planting that flag, and conveying to the rest of humanity the thrill of it all.

Yet the Sealab program, for all its successes during the 1960s, was allowed to (pardon me) sink. Since then, despite some noble, if relatively low-budget, efforts, it’s become clear: Where manned undersea ventures are concerned, we are not boldly going anywhere, or not nearly as boldly as once envisioned, anyway. History suggests that we have a habit of throwing in the towel when it comes to the glorious but perpetually underfunded and underappreciated field of manned sea exploration.

The kind of public interest and unbridled enthusiasm that has long sustained the space program and NASA’s multibillion-dollar budgets has never materialized for like-minded quests into the ocean. Last year’s near closure of the world’s only sea base was the latest case in point. If you can’t name this unique, American-run undersea outpost, you are not alone, and that’s at least part of the problem. It’s called the Aquarius Reef Base, and for the past two decades, this school-bus-sized structure has been operating a few miles south of the Florida Keys and a few fathoms below the surface.

From its beginning Aquarius has typically had to squeak by on less than $3 million a year, sometimes much less—a drop in the fiscal bucket by space program standards. (NASA’s estimated cost of a single space shuttle launch, for example, was $450 million.) Then last year the National Oceanic and Atmospheric Administration, which owns Aquarius, decided to pull the plug on the base. An organized effort to save Aquarius created an unusual surge in media and other attention—not major front-page headlines, to be sure, but there was at least a discernible spike.

By the end of the year, the base had been spared, at least for the time being, though a long-term solution to keeping it open is still being worked out. The point is that for all the Champagne bubbles about saving Aquarius, it was soon clear that this was just a chapter like a lot of others in a decades-long history of false starts and unfulfilled dreams.

Around the same time the space program was getting off the ground, in the late 1950s and early 1960s, a revolution in manned undersea exploration was in the making. Most Earthlings weren’t aware of it, but U.S. Navy scientists and divers were quietly working to figure out whether there was a way around conventional diving limits, in terms of both depth and duration. At the time, it was considered extreme to dive much deeper than, say, 200 feet—and the deeper the dive, the shorter the stay. (We’re talking mere minutes.)

As is well-known, NASA had to do thorough animal and lab testing before subjecting human astronauts to gut-squishing G forces, weightlessness, and the whole laundry list of physical and psychological challenges that come with traveling into space. The list is at least as long for bodies exposed to the depths, pressures, darkness, cold, currents, and creatures of “inner space,” as some like to call the subaquatic realm. (Just ask Scott Carpenter, the Mercury astronaut and second American to orbit the Earth. He also took part in Sealab.)

What emerged from the Navy experiments was a game-changing method called “saturation diving,” for reasons having to do with how the gases you breathe are absorbed into your blood and tissues. “Sat diving” gave divers a way to acclimate to a particular depth and then stay indefinitely—for hours, days, and even weeks, provided they had some kind of shelter or base, with a heated, dry interior pressurized to match the pressure of the seawater outside. With this setup of equalized internal and external pressures, a hatch in the belly of an undersea base could remain open. Water would stop at the brim and the aquanauts living inside could don their dive gear and leave the base at any time of day or night, as they can at the Aquarius Reef Base. Marine researchers or anyone else could have the kind of unfettered access to the seabed that Jane Goodall had to the jungle. (There was just one thing: Immediate surfacing from a saturation dive at any substantial depth would kill you. A carefully orchestrated, gradual return to the surface is essential—one of the reasons it’s sometimes best to stay inside the protective confines of a submarine or other such submersible craft.)

Among the first to pick up on the exploratory possibilities of saturation diving was Jacques-Yves Cousteau, who was already well-known as an ocean explorer and the co-developer of modern scuba. Having learned about the Navy experiments, Cousteau went ahead and staged some early demonstrations of sea dwelling, in 1962 and 1963. In the summer of 1964, the U.S. Navy put its concept of saturation diving to a deeper, more difficult test: A 40-foot-long, cigar-shaped steel habitat called Sealab I was placed nearly 200 feet below the surface off the coast of Bermuda. Four aquanauts were going to try to live and work for three weeks down there—an unprecedented undertaking.

This was followed up in 1965 by Sealab II, a larger, heavier, more refined habitat resembling a railway tank car, with room for 10 aquanauts at a time. (And, incidentally, there was a role for a Flipper-esque trained porpoise.) Sealab II was placed a little deeper than its predecessor and in more challenging waters (colder, darker) at a site off the coast of San Diego.

A few months before the moon landing in 1969, Sealab III—an elaborate, 300-ton, expanded version of Sealab II—looked as though it might be the undersea counterpart of the moon shot. The lab was lowered to more than 600 feet off the coast of Southern California, a remarkable dive depth even today. Several dozen aquanauts were going to take turns living in the lab, with a variety of military and marine science projects on the agenda, including exploratory saturation dives from the lab down to 1,000 feet.

This might have been a true turning point for manned undersea exploration, the project that finally secured a needed foothold in the national zeitgeist. Jacques Cousteau’s son Philippe was lined up to film the mission.

But just as Sealab III was getting under way, a tragic confluence of human error and technical difficulties—including some malfunctioning breathing gear—led to the death of an aquanaut who had taken part in the successful Sealab II missions. After an investigation, the Navy canceled the program. The Sealab crew—Navy divers and marine scientists alike—found this utterly perplexing. Three astronauts had died in a launch pad fire inside the Apollo 1 capsule two years earlier and that didn’t spell the end of the space program. Imagine the national uproar if NASA had announced that it was abandoning the moon mission. But Sealab’s demise was unceremonious, with scarcely a peep on TV or a headline in the newspapers—another measure of how manned undersea exploration still hadn’t caught on, its low profile partly a function of low budgets.

After Sealab, in the 1970s, there actually was talk about creating a “wet NASA”—an independent civilian agency like NASA, but funded to pursue manned and other undersea activities. The idea gained credence in 1969 with the publication of “Our Nation and the Sea: A Plan for National Action,” a gung-ho report from President Lyndon Johnson’s blue-ribbon commission on ocean issues. But responsibility for undersea missions was instead put under the Commerce Department and the wide umbrella of the National Oceanic and Atmospheric Administration, which was formed in 1970 from a smorgasbord of government agencies and departments.

In the early 1980s, as the space shuttle program was taking off, the United States considered reviving the Sealab concept in the form of a mobile habitat to be called Oceanlab, which was designed to roam inner space like Jules Verne’s mythical Nautilus in 20,000 Leagues Under the Sea. Oceanlab would be capable of reaching depths up to 1,000 feet and equipped to release saturation divers wherever they wanted to explore. As you have no doubt guessed, the Oceanlab plans were shelved, but it’s interesting to imagine what Oceanlab, a kind of marine version of a space shuttle, might have done for the cause of manned undersea exploration—not to mention the multiple habitats, both fixed and mobile, that Johnson’s commission had recommended.

A number of countries came up with Sealab-style prototypes, as did a few eager entrepreneurs, sometimes with public funding. These efforts included Hydrolab, a little habitat with room for three or four aquanauts. NOAA bought Hydrolab in the 1970s, and it would go on to house several hundred scientist-aquanauts over the course of many missions in the waters off Bermuda and St. Croix in the U.S. Virgin Islands. But these private groups—including the team of Jacques Cousteau, who was as great a pitchman and fundraiser as anyone—would find sea dwelling and exploration a tough business to pursue, especially without a government-primed infrastructure and market like the one that evolved for space travel. The situation was something like tech mogul Elon Musk trying to launch SpaceX without the benefit of a space station or the many trails NASA blazed with its billions.

In the mid-1980s, NOAA did have Aquarius built as a replacement for Hydrolab. While Aquarius was considerably larger and a more state-of-the-art habitat, its annual operating budget remained paltry, usually between $1 million and $3 million. By the time NOAA proposed closing the base last year, the agency was already putting a lot more of its resources into robotic approaches to undersea exploration, making it even less likely that people will boldly go where no man (or woman) has gone before.

Still, not everyone has given up on the idea. The renowned French architect Jacques Rougerie has for a decade been leading a consortium in the development of SeaOrbiter, a vessel resembling an aquatic cousin of the Starship Enterprise. I’ve been told the project is coming along and that its unusual design—vertically oriented, like a buoy, to float at the surface and drift with the currents—includes the necessary setups for saturation divers. And coming this November, Fabien Cousteau, the eldest grandson of Jacques Cousteau, plans to begin “Mission 31,” during which he and half a dozen others will live at the Aquarius Reef Base for an attention-getting 31 days. They’ll be commemorating the 50th anniversary of Fabien’s late grandfather’s experimental habitats in the Red Sea—those structures became the setting of a picture-laden book, World Without Sun, and an Oscar-winning documentary of the same name.

In many ways “Mission 31” will be like a sequel to last year’s Aquarius mission, which helped save the base by showcasing its role in studying marine life and the exploratory advantages—and the thrill!—of being an aquanaut. I suppose if anyone can jump-start the sputtering age of manned undersea exploration, a Cousteau armed with IMAX and live-streaming cameras can. But if history is any guide, I wouldn’t hold my breath.

More from Slate’s series on the future of exploration: Is the ocean the real final frontier? Why are the best meteorites found in Antarctica? Can humans reproduce on interstellar journeys? Why are we still looking for Atlantis? Why do we celebrate the discovery of new species but keep destroying their homes? Who will win the race to claim the melting Arctic—conservationists or profiteers? Why don’t travelers ditch Yelp and Google in favor of wandering? What can exploring Google’s Ngram Viewer teach us about history? How did a 1961 conference jump-start the serious search for extraterrestrial life? Why are liminal spaces—where urban areas meet nature—so beautiful?

This article arises from Future Tense, a collaboration among Arizona State University, the New America Foundation, and Slate. Future Tense explores the ways emerging technologies affect society, policy, and culture. To read more, visit the Future Tense blog and the Future Tense home page. You can also follow us on Twitter.