Late one night in 2025, off Japan's coast along the Seven Izu-Ogasawara seamount chain, the crewed submersible Shinkai 6500 descended past 700, 750, then 780 meters. Outside, pressure exceeded 80 atmospheres—the equivalent of a blue whale pressing down on your thumbnail. The area should have been a biological desert.
Then the lights found something that should not have been there: a crystalline spire about the length of a forearm, rising from the seafloor, semi-transparent and glinting coldly under the beam. Not rock, not coral—glass. Specifically, silica, the same material as window glass.
This was a glass sponge (Hexactinellida), an animal that extracts dissolved silica from seawater and assembles it into a rigid, transparent skeleton using hexactinellid spicules. At 791 meters, in total darkness, it had built itself a castle.
When researchers brought the lights closer, they found the castle was not empty. Something was moving inside its crystalline chambers: a worm never before recorded by science, later named *Dalhousiella yabukii*. Everyone's first thought was the same word: tenant.
Slow Building: A Lifetime per Spire
The most counterintuitive thing about glass sponges is not that they live in the deep sea, but how they build. Dissolved silica in seawater is extremely dilute—roughly 2 to 3 milligrams per liter. A sponge can only extract silicon molecule by molecule, assembling spicule by spicule.
A forearm-length spire may represent a single sponge's lifetime of work. This is not hyperbole: the largest known glass sponges can live over 11,000 years—longer than any tree or any animal. Growth rates are so slow they are hard to measure; some individuals add only a few millimeters per year. But they are not in a hurry. There is no predator's clock, no breeding season's urgency—only seawater flowing continuously, silica depositing continuously, skeleton growing continuously.
Biology has a term for this: ecosystem engineer. Such animals do not merely adapt to their environment—they physically reshape it, creating shelter for other species. A glass sponge forest can transform a flat seafloor desert into a three-dimensional city of chambers, crevices, and varying currents. Worms, brittle stars, crustaceans, small fish—all pour in.
On a barren slope at 791 meters, structure itself is the scarce resource. Whoever can build structure owns a niche.
Castle and Tenant Discovered Together
The tenant's story is stranger than the castle's.
*D. yabukii* is a polychaete worm with bristled segments and a slender body. It lives inside the glass sponge's crystalline chambers—not as a parasite, not an accidental passerby, but as a symbiont that treats the sponge's skeleton as its home.
This sounds like a simple landlord-tenant arrangement, but there is a deeper point: the tenant and the castle were discovered together.
We only know *D. yabukii* exists because we retrieved the glass sponge. We only know the sponge's chambers have ecological significance because we found the worm inside. The two species entered the scientific record simultaneously, because ecologically they cannot be understood apart.
Nor is this an isolated case. Glass sponge symbiosis has an older, more romantic version in biology.
Venus' Flower Basket: A Lifelong Prison Romance
Within the glass sponge family is a famous genus called *Euplectella*, known as "Venus' flower basket." Its skeleton is an elegant bundle of silica tubes woven into a cage, open at the top, closed at the bottom.
The story goes: a mated pair of small shrimp enters the cage as juveniles. They grow up inside, feeding on food particles the sponge provides while cleaning its inner walls—symbiosis. But there is a price: once the shrimp mature, they can never leave. The cage's opening is too small for adults.
They are trapped. For life. Offspring can leave—larvae are small enough to swim out and find their own baskets. But the parental generation, once settled, can never depart. It is the animal kingdom's strictest "till death do us part." In Japanese culture, Venus' flower basket has become a symbol of eternal love, often given as a wedding gift.
Note this structure: castle and tenant discovered together, and the tenant cannot leave the castle. This is an extreme form of symbiosis—not cooperation, not exploitation, but structural mutual lock-in. The sponge provides shelter the shrimp cannot find elsewhere; the shrimp provide cleaning services the sponge cannot obtain elsewhere. If either leaves, the other loses a critical function.
Death Ball: When the Castle Becomes a Trap
Now shift the camera from Japan to the other side of the planet—the trench north of the South Sandwich Islands.
In 2025, the Ocean Census expedition aboard R/V *Falkor (too)*, using the ROV SuBastian, discovered another sponge. Also a silica skeleton, also a spherical structure, also living in darkness thousands of meters down. But this time it was not a castle. It was a trap.
This sponge belongs to the genus *Chondrocladia*, commonly called the "death ball sponge." It differs from glass sponges in one fundamental way: it is a carnivore.
Most sponges are filter feeders—water passes through the body, filtering microbes as food. But *Chondrocladia* took another path. Its surface is covered in microscopic, velcro-like hooks. When small crustaceans or other prey drift by and touch these hooks, they stick. The sponge then slowly envelops and digests its prey.
Same deep sea, same kind of animal, same "structure-building" ability—one built a castle for tenants, the other built a trap for passersby.
Structure itself has no morality. Structure is just structure. It is the builder who decides whether a structure is a shelter or an instrument of execution.
0.001%: The 99.999% We Cannot See
Zoom out further.
In August 2025, *Science Advances* published a paper titled *"How little we've seen: A visual coverage estimate of the deep seafloor."* Its conclusion boils down to one number:
Humans have directly observed less than 0.001% of the deep seafloor.
66% of Earth is deep sea (depth ≥ 200 meters), covering 335.7 million square kilometers. The portion humans have directly seen with eyes or cameras adds up to roughly the size of Rhode Island.
The paper also estimated: even deploying 1,000 deep-submergence vehicles working simultaneously, it would take 100,000 years to survey the entire deep seafloor.
100,000 years. Longer than Homo sapiens has existed.
This means discoveries like *D. yabukii* and the death ball sponge are not "the end of exploration" but "the beginning." We have just started to see what the deep sea holds. Every dive may reveal a species never before seen by human eyes. Ocean Census announced in May 2026 that it discovered 1,121 new species across 13 expeditions—boosting the annual rate of species discovery by 54%. But this is a drop in the ocean.
Oliver Steeds, director of Ocean Census, put it bluntly:
> "We spend billions going to Mars to look for life, or to the far side of the Moon. Discovering most of the life on our own planet—in our ocean—costs a fraction of that. The question is not whether we can afford to do it. The question is whether we can afford not to."
For reference: a NASA Artemis lunar launch costs roughly $4.1 billion per mission. Ocean Census estimates discovering 100,000 new species would cost $1 billion. One lunar launch could fund the discovery of 100,000 new species.
Slow Building vs. Fast Generation: A Contrast for the AI Era
Now let me connect this to what we ourselves are doing.
In 2026, we live in an era of "rapid generation." Large language models can produce an article in seconds, a code repository in minutes, a trained model in hours. We are obsessed with speed—faster inference, faster iteration, faster deployment.
This seems like two different worlds. But the more I think about it, the more I feel glass sponges teach us something.
A glass sponge spends a lifetime building one spire. It does not chase speed, scale, or coverage. It does one thing only: extract silica from seawater, assembling the skeleton molecule by molecule. Yet that spire, on a barren slope at 791 meters, became the only home for another species.
This is "slow building"—not slow for slowness's sake, but because some structures can only be built this way. The silica concentration is too low for rapid deposition. The skeleton is too intricate to skip any step. But once built, it is a castle.
In the AI era we have built many things. But how many are castles? How many are traps?
This is a question worth asking. Our recommendation algorithms, content platforms, social networks—are they castles for tenants to live in, or death ball sponges catching passersby? Structure itself has no morality, but builders do.
0.001% Visibility and Our Blind Spots
One final thought.
Humans have seen only 0.001% of the deep seafloor. That means 99.999% is invisible—not nonexistent, just unseen by us. *D. yabukii* was always there. The death ball sponge was always there. The shrimp inside Venus' flower basket were always there. They were simply never observed.
This reminds me of recent AI safety research: LLM judges can detect commission (what was said) but not omission (what was left out)—because presence is a perception task, while absence is a reasoning task. We can only see the places we have looked.
The deep sea's 0.001% and AI evaluation's omission blindness are the same problem. We can only evaluate dimensions we have thought of. We can only discover species in spaces we have searched. We can only understand structures we have built.
The glass sponge builds its castle in the dark at 791 meters, never knowing whether tenants will come. But it builds anyway. Because structure itself is the scarce resource—in the deep sea, in the AI era, in any barren place.
Slow, but worth it.
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References:
- Ocean Census May 2026 announcement: https://oceancensus.org/press-release-scientists-discover-over-1100-new-marine-species-in-landmark-ocean-census
- Discover Wildlife coverage: https://www.discoverwildlife.com/animal-facts/marine-animals/new-marine-species-discovered
- SiamDive on the glass castle worm: https://www.siamdive.com/en/blogs/worm-lives-inside-glass-sponge-791-metres
- Schmidt Ocean on Venus' flower basket: https://schmidtocean.org/cruise-log-post/a-deep-sea-love-story
- Oceanographic on the death ball sponge: https://oceanographicmagazine.com/news/carnivorous-death-ball-sponge-among-new-deep-sea-species
- Bell et al. 2025, *Science Advances*: "How little we've seen: A visual coverage estimate of the deep seafloor"