SOMEWHERE OVER THE PACIFIC OCEAN. Over the course of four decades, Alvin, the famed submersible that explored the sunken Titanic and discovered black-smoker hydrothermal vents, has made more than 4,400 dives, but each time only three people are treated to a firsthand view. In all, 2,500 people have taken the plunge. Courtesy of my former life as an oceanographer, I know a grand total of five of them.

Glimpsing the sea floor is a rare privilege. Human eyes have touched only 5 percent of it; we know less about the planet’s deepest reaches than we do about the surface of the moon. On every submersible dive, a new species is discovered.

For us surface-dwellers, the growth forms that have evolved to survive the intense pressure and unbelievable blackness of the ocean floor are incredibly alien. Their weirdness extends beyond their physical appearance to metabolisms fueled by strange chemicals, or slowed to match the cold, barren environment. And yet, as they float through our TV screens or curve across still photos, they show a grace shaped by the liquid medium that envelops them.

No wonder we were so eager for news last week from James Cameron’s solo journey to the Challenger Deep, the very bottom of the Mariana Trench. Not only did the famed director promise to assemble his footage into a documentary, but his feat of engineering and audacity also forced a fundamental point: with some (admittedly large) financial clout, anyone can reach the sea floor.

Although, like the commercialization of space flight, deep-sea ventures for the common man are a long time coming, their possibility ties the ocean ever closer to the realm of personal experience. This is the first step toward recognizing the intimate role the deep ocean plays in maintaining Earth as we know it.

That realization couldn’t come sooner.

The day after James Cameron surfaced from his record-setting dive, a new report was released on the state of the seafloor in the Gulf of Mexico.

Six months after the Deepwater Horizon’s 200 million gallon oil spill, a deep-diving scientific team (using manned and unmanned submersibles, including Alvin) found a benthic community struggling to recover. Miles from the spill source, they discovered corals choking on oil residues and pale brittle stars clinging to life, the sickly remnants of a once-thriving site.

At the surface, fishermen are preparing to mark the second anniversary of the spill on their calendars, and wondering what this season’s catches will show. They doubt the reality will match BP’s rosy advertisements of recovery. After all, hasn’t it been 23 years since the Exxon Valdez spill in Alaska, where a damaged herring fishery is just beginning to turn the corner?

For most of us though, out of sight is out of mind. Once oiled seabirds stopped washing up in media coverage, we started forgetting about the largest oil spill in American history.

But deep below the sea surface, life is slow to forget. Like oil residues, bottom trawling (the marine version of clear-cutting responsible for our catch of flounder, cod, and scallops), ocean warming, acidification and various other types of pollution cause lasting damage.

The damage we do to the deep ocean is also damage to its capacity to support human life. The deep ocean is the final recycling point for many nutrients, shunting them back to the surface to fuel life anew. And the deep ocean also slows global warming: cold, polar water traps human carbon dioxide emissions, and then sinks to the sea floor, trapping the greenhouse gas for up to a thousand years.

The deep sea also holds the promise of new technology — compounds that could treat bacterial infections or cure cancer — and new resources in minerals and energy deposits that remain untapped. Of course, as Deepwater Horizon demonstrated, accessing these resources comes with risks — risks that increase with depth and that we cannot fully understand so long as we do not fully comprehend life in the deep sea.

Skimming homeward over the Pacific Ocean aboard a giant Boeing 747, wide-awake despite the dimmed cabin and late hour, I briefly wished for a window seat. Of course, at our altitude, I wouldn’t be able to make out anything below. But I could imagine: the silhouettes of islands, the pinpricks of ships’ lights, James Cameron’s boat perched above the Mariana Trench and the invisible realm of life that lay thousands of meters below.

One day, I might have an opportunity to see that life for myself. But until then — and because I’m not sure how much longer that life will be around — I’ll have to settle for documentaries.

Send comments, questions or tell Holly she’s gone off the deep end via email at hollyvm “at” stanford “dot” edu.