This is a story from the past—though not the distant past; it is recent enough that it feels like only yesterday.

Humanity fell in love with two "boundless realms." One was the pitch-black cosmos stretching out overhead; the other, the ultramarine deep sea lying far beneath our feet. Both were unknown frontiers that no one had truly set foot in—not in the deepest sense of the word.

The setting is 2026. One of these romances has borne fruit in astonishing ways. Rockets soar into the heavens time and again, returning quietly to solid ground. Meanwhile, that invisible intelligence known as Artificial Intelligence evolves day by day, growing wiser as if by magic.

Yet, a strange silence hangs over the other romance. Even though humanity has sent probes to the very edge of the solar system, countless mysteries still slumber at the bottom of the ocean on this very planet we call home. It is said that we have mapped the surface of Mars in greater detail than the depths of the Mariana Trench.

Why is there such a disparity in the pace of progress between these two "challenges into the unknown"? This is the record of a small journey to unravel that mystery.

## Chapter 1: A Misconception About Weight

Many people assume that because the deep sea is far closer than space, reaching it must be easy. But this simple intuition is the first pitfall.

Space is made of "nothingness." A vacuum is a realm where pressure is virtually zero. The forces a rocket or spacesuit must withstand are merely the pressure pushing outward from within—roughly one atmosphere. In a sense, the vessels need only be slightly sturdier than a soda can.

The deep sea, however, presents the exact opposite scenario. At the ocean floor—nearly 10,000 meters down, where not a single ray of sunlight reaches—the weight of the water exerts a crushing force on the human body that far exceeds 1,000 atmospheres. The weight involved is roughly equivalent to having an entire compact car resting on an area the size of a thumbnail. Creating a vessel capable of withstanding forces that crush it evenly from all directions—pushing inward from the outside—presents a far more daunting engineering challenge than building one that withstands pressure pushing outward from within.

The intuitive notion that "proximity implies simplicity" crumbles instantly when faced with the laws of physics.

## Chapter 2: A Place Beyond the Reach of Light and Sound

Space possesses another characteristic that works to humanity's advantage: because it is a void—"nothingness"—light and radio waves can travel billions of kilometers with almost no obstruction. Telescopes capture galaxies hundreds of millions of light-years away, and the Voyager probe, even now that it has left the solar system, continues to communicate with Earth via faint radio signals. Space is a remarkably hospitable stage for the kind of "dialogue via light and radio waves" at which humanity excels.

The ocean is the exact opposite. Sunlight virtually vanishes after a descent of just 200 meters; beyond that point lies an endless night. Radio waves, too, are easily absorbed by water, failing to travel even a few meters. To communicate with a deep-sea probe, one cannot rely on light or radio waves, but must instead turn to sound echoes—sonar—a method that is slow and capable of conveying only a minimal amount of information.

If space is a place where voices carry, the deep sea is a place where voices are lost.

## Chapter 3: The Economics of Speed

There is another reason that is often overlooked: the cost of trial and error.

The true essence of rocket development lies in the staggering speed of its iterative cycle—launching and landing, analyzing failures, and rebuilding. Artificial intelligence evolves according to this same logic. New models can be tested repeatedly using data and computation, allowing us to create a "today" that is smarter than "yesterday" powered solely by software. Both space exploration and AI have gained a distinctively modern advantage: the ability to take on challenges cheaply, quickly, and repeatedly.

Deep-sea submersibles, however, are a different story. Building a single vessel takes years and vast sums of money, and even a single dive entails meticulous preparation and significant risk. They are not the sort of thing where you can simply try out the "next version" if something breaks. The difference is akin to that between releasing a new smartphone every year and rebuilding a bridge every year.

While space exploration and AI race ahead, driven by the modern rules of "iteration and improvement," deep-sea exploration remains quietly left behind, bound by the old rules of the "one-shot gamble."

## Chapter 4: The Deep-Sea Fish Named Fear

Physics and economics alone cannot fully explain everything. There remains another abyss—invisible to the eye—known as the human heart.

Throughout history, human narratives have portrayed the sky as a place of longing—think of sun gods, angels, and ascensions. Even rockets are often given the names of deities. Looking upward has always been a story of hope.

In contrast, the depths of the ocean have often been depicted in mythology as the dwelling place of the unknown. While Japanese folktales speak of beautiful underwater kingdoms like Ryūgū-jō, a quiet dread of the darkness beyond human understanding always lurks in the depths beneath them.

When we look up at the sky, our spirit of adventure ignites; when we peer into the ocean depths, we instinctively flinch. Perhaps this shared, nameless inclination of the human heart plays a subtle role in the sluggish pace of investment in deep-sea exploration.

## Final Chapter: Another Universe

That said, this story is not without a ray of light. "Seabed 2030," an international project led by The Nippon Foundation and GEBCO, continues to make steady progress toward its goal of completing a detailed map of the global seafloor by 2030. Unmanned submersibles quietly continue their descent into the darkness of the deep ocean. It may lack the glamour of space exploration, but it represents genuine progress.

In a sense, space might be a mirror reflecting humanity's "future," while the deep ocean serves as a mirror reflecting our "origins"—revealing where life itself began.

Ironically, the very technology that turned humanity's gaze toward the cosmos—AI—may one day provide the key to gently unlocking the door to the deep ocean. We look toward a future where autonomous submersibles traverse places humans cannot reach, exploring on our behalf.

In truth, another universe lies slumbering right beneath our feet.

Perhaps we simply lack the full measure of courage needed to open that door.