The following ideas and connections are not all of my own making. This post comes from discussion here, on Reddit and on /a/. I’ve added a few thoughts and insights of my own, but this is mostly me putting some broader discussion on plausible theories together into a readable format. Mainly, we’re looking at the last two pages of chapter 906 and how they connect to other bits of lore and unsolved mysteries in One Piece’s word. Let’s see if I can make this all make sense. Images provided in the spoiler tags under each section, let’s go.
Observation: the room under Marie Geoise containing the straw hat has three key features we’re going to use for this set of theories. 1) There are seven openings in the wall. 2) There’s a symbol similar but different to the World Government’s flag carved into the wall above the openings, and 3) It’s cold enough inside that everything has iced over. All of these and the presence of the straw hat are important.
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Let’s start making connections. That not-quite World Government symbol on the wall looks familiar, and we have seen it before. It was on Oars’ clothes, which we can assume he was wearing at his time of death around five hundred years ago. That means its old. Not Void Century old, but old none the less, possibly a prototype for the current design. And it also means that Oars probably worked with the World Government when he was alive. We’ll be coming back to that.
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We now know that whatever is in the straw hat room under Marie Geoise is at least five hundred years old, maybe older.
Alright, now we know there’s some pretty ancient history in here, let’s tie it to the moon. First we have to assume the straw hat here and Luffy’s straw hat are connected. Not a huge leap, considering their identical designs, save for the size difference, and Luffy’s wanted poster being brought to the Marie Geoise hat.
I feel safe in extrapolating that Luffy’s hat has some connection to the Will of D. Shanks isn’t a D. (that we know of) but Gol D. Roger, at this time the oldest known owner of the hat, definitely was. It’s been theorised for some time that the people of the D are connected to the moon, but here’s a quick refresher on that for the uninitiated: Enel found the ruins of an ancient civilisation on the moon in his cover story - wall carvings in these ruins showed humans with angelic wings, similar to the Sky Islanders. It could be the exaggerated proportions of the carvings at work, but the moon people’s wings seem smaller than the Sky Islanders’, indicating the trait could be minimised or bred out completely over time. The theory goes that that people of the D are descendants of the moon people who totally lost their wings, unlike the Sky Islanders, who only had them reduced. What connects the two? A theory that the D. isn’t an initial but a glyph representing a half moon, and the cover of Volume 27, which depicts Luffy with a pair of sky islander angel wings behind him, winking at the reader. It could just be a bit of whimsy for the cover, or it could be some of the most subtle foreshadowing ever.
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So the Will of D relates to the legacy of the moon people and the small straw hat relates to the Will of D. That is all speculative, so dismiss it if you like, but the pieces do seem to fit together.
New factor: the Oharan globe. Now this thing, seemingly depicting One Piece’s earth with six satellites, one of which has a sub-satellite of its own. So seven other celestial bodies depicted with One Piece’s earth. I think we can dismiss out of hand the idea that the planet actually has six moons. No image of the night sky has shown more than one, and every mention of the moon is singular only, no plurals. So it’s just a geocentric interpretation of the solar system then? Maybe. But maybe not.
One Piece’s world has only one moon. But what if it used to have more? The Oharan scholars are archaeologists before anything else, not astronomers. They deal in ancient history, with a specific affinity for things like the Void Century and other records that have been lost to time. Their infamous globe could represent not how the world is but how it was, based on what they found in their research.
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Assuming the above is correct, the world of One Piece had seven moons (counting the sub-satellite, since they are a scientifically plausible thing) in ancient times. At least one of these moons was populated. We established before that the Will of D and the small straw hat represent the legacy of the moon kingdom, but what if we instead say they represent the legacy of a moon kingdom instead. Then they and the Sky Islanders can both be descended from moon races - different moon races - without having to get into awkward wing size genetics, and the Volume 27 cover art still works to associate the two as being descended from moon people, just two different races of moon people now. One moon being populated leads to two, and from two it’s not such a leap to say they all were. Seven moon races. Seven openings in the wall in the room under Marie Geoise. The straw hat represents a moon race. There’s a straw hat in one of the wall openings. Ergo, each opening represents a moon and a moon race.
EDIT: Theackwardstation suggested another excellent piece of evidence that there were several moons in the past: Duke Dogstorm calls Sulongs the Minks' "true form," which is a little odd for something they only have the opportunity to be once or twice a month. On a world with at least six visible moons, however, it's not hard to believe they could have been able to stay Sulong most if not all of the time, and only then does the "true form" statement really fit. Additionally, given how wild and dangerous they would have been in a near-constant Sulong state, it could explain the reputation the Minks have managed to get for being killing machines despite no one in the present day really knowing them and them being actually pretty friendly in person.
Logical enough so far.
To say the World Government destroyed six out of seven moons though, is a bit more of a stretch. We have to extrapolate and guess more for that. The World Government did something bad during the Void Century. We know it has to be pretty shocking, since they’re pretty open about things like genocide and slavery today. Perhaps they destroyed six out of seven moons and genocided the moon races to the best of their ability. Now that’s the kind of enormous-scale atrocity you’d want to keep out of the history books.
But why?
This is where I feel most likely to be wrong, but it could simply be the tides. Seven moons would play havoc with them and cause all kinds of destructive effects, and if the World Government wanted to tame the seas that would be an obstacle.
Or it could even have been just a survival thing. It gets unscientific here, but ‘planet aligning’ events are a common trope, and a seven moon planet could have the same thing happen with aligning moons. Seven moons in a row could have created a tide high enough to create an apocalyptic, biblical flood. In such an event, the only safe places would be Fishman Island, equipped to survive underwater, Marie Geoise, 10,000 meters above sea level, and with the moon civilisations. It’s possible, even, that the Redline was created for the sole purpose of surviving this high tide, employing the help of the Continent Pullers.
Wait why’s that a plural? For the sake of this post I’ll be considering Continent Puller a separate classification to giant. Their increased size and distinctive horns (more on those traits later as well) set them apart. It could be a mutation unique to Oars’ line, or it could be a whole separate but extinct species, but they are definitely a different thing to your average giant and deserve to be considered as such.
We also know that Oars’ ‘Continent Puller’ title isn’t just empty hype. Official art for One Piece Magazine shows Oars in his prime pulling a whole landmass behind him with chains. The epithet seems to be a real indication of strength.
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So we have a group of super-Giants with strength enough to literally change the face of the planet, we have the corpse of one of these turn up wearing an ancient World Government insignia, and we have a World Government with a very convenient flood-proof holy land atop a 10,000m high pangeaic continent. There pieces all line up, right? This makes sense.
As an aside that won’t tie into much else here, interesting that the Ancient Weapon Pluton is a ship. Maybe it was an ark the early World Government used to survive the lunar floods while the Redline was under construction. It fits with the theory that Noah is Pluton, especially considering Noah’s placement, as if it was abandoned and scuttled to sink to Fishman Island when the World Government disembarked from it a final time onto their new holy land. And if the seafaring races that formed the World Government had one ancient weapon, and we know the underwater races of Fishman Island held another in Poseidon, does that mean the moon races must have had Uranus, to keep a balance of power? And to go a little further down that line, symbolically, Poseidon is named for an ocean god and Uranus for one of the skies. Pluton, for a god of the underworld, doesn’t fit as cleanly, but the genocidal nature of the World Government helps it a bit.
So the World Government conquered the tides once, working with the Continent Pullers and abandoning their Ancient Weapon in the process. Then they got cocky, got caught up in their taste of godlike power and decided to put a stop to the lunar floods once and for all.
It’s hard to say exactly how they did this. Perhaps Uranus was captured and turned on the moon races. Maybe it also goes back to the Continent Pullers. Could something like Fishman Island’s ancient and treasured Energy Steroid have been used to bump their power up from ‘continent pulling’ to ‘planetoid destroying?’ Again, this area of the theory is pretty vague and subject to change as most information comes up.
Regardless, the Moon War was fought and won in the Void Century, each of the moon kingdoms’ kings caught and killed, and a space made for each of them under Marie Geoise, each space with a memento of the moon king on top, such as a straw hat. Then all record of the Moon War was erased. The Continent Pullers, however many of them there actually were, were betrayed by the World Government and killed. This may have been just for the cover up, but it could also be because the World Government lost control of them after souping them up to fight the Moon War. Oars is said to have frozen to death, yes, but his body shows signs of numerous serious injuries, and it’s hard to tell what’s bits that just didn’t preserve properly and what happened before he died. Or being stripped of protective gear and left for dead on the Ice Continent was his execution. Either way it’s important that he died there specifically, and we’ll talk about why in a minute.
First, let’s talk about the Continent Pullers being made victims of the Void Century, because there are a couple of little symbolic nods in that direction to think of. The first was Volume 48’s cover. The same way Luffy was framed in front of Sky Islander wings on the Volume 28 cover, Luffy is here framed in front of Oars’ fangs, making it look like he has horns. Not unlike (but also not totally like, to be fair) the distinctive horns the Continent Pullers all have. Similarly, Little Oars Jr, the descendent of the original Oars and our only known living Continent Puller, was made sympathetic through his friendship with Ace. Ace, of course, carries with Will of D and even weaved Oars Jr. a straw hat in the Wano style. Horns on Luffy, a straw hat on Oars Jr. Not exact matches in either case, but similar enough to connect the two. As with the Sky Islanders, an association is being built here.
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Alright, I wanted to talk about the significance of Oars dying in an icy place, and not just because Thriller Bark couldn’t have happened if he didn’t. And I hadn’t got to why it’s important that the room under Marie Geoise is a freezer yet either. But to talk about that, first we have to talk about Punk Hazard.
The World Government was doing giantification experiments on Punk Hazard, among other things. We’ve been told that repeatedly. They want giant soldiers because the strength of Giants is unmatched, and they would probably want to move from there to Continent Puller soldiers, knowing how effective they were in the Moon War. Wanting to go from Giants to Continent Pullers isn’t conjecture either - the warning signs outside Punk Hazard feature a skull with distinct Continent Puller horns on it, and Luffy’s group discovers a skull too large to belong to a giant. We can’t see it in full, but it seems to have a curved horn coming off it as well. Continent Pullers were obviously part of the plan. There were Continent Puller remains on the island. If they worked with and then killed the Continent Pullers it makes sense that the World Government would know how powerful they are and be trying to recapture that power, and where to find remains for genetic material to work with.
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As another aside, Kaido is a Giant sized creature with Continent Puller horns. He’s referred to in ever case as a creature, not a human, and has the kind of nigh-invulnerability you would probably want a soldier to have, and the nihilistic attitude of a man traumatised by an upbringing of horrific genetic experiments. And would it work thematically for a human lab rat to have the affinity for animals that he has? Of course. Kaido could very well be a failed and escaped attempt to genetically engineer new Continent Pullers.
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Anyway, we see that Punk Hazard has a large cold storage area full up with the corpses of Giants. The World Government is savvy enough to store Giant bloodline elements they find. It makes sense then that Oars was intentionally left to die or killed and then moved to somewhere he would be preserved for later research. And then forgotten about. Or the lab was abandoned after a Punk Hazard style accident until Moria found it. Or Oars escaped the attempt on his life and fled into the icy wastes where he died alone and lost. There’s honestly a few different ways it could have played out. But the bottom line is that the World Government, going back hundreds of years, is savvy enough to store the remains of powerful entities so they can try to genetically recreate them later.
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So perhaps the room under Marie Geoise isn’t just a symbolic shrine of victory over the moon kings. Maybe each of those openings in the wall doesn’t just have a keepsake from the fallen enemy inside, but the actual corpse of the enemy. The crypt is kept frozen to preserve the bodies for later use.
And what later use is that? Well we were actually just told exactly what. Dr Vegapunk’s last two big projects were cyborging Kuma and creating the Pacifistas. The Pacifistas are human corpses combined with cyborg parts to make humanoid weapons. The cyborging of Kuma is interesting largely because Kuma has been made into a mindlessly obedient killing machine, but hasn’t been stripped of his Warlord title. He’s an experiment, to see if such a machine can still bring the intimidation factor the Seven Warlords require to keep their rank. See, the World Government would want something with that fear factor to keep their member states in line and maintain the power balance with the Emperors, but they’d also love something more predictable and easy to control than the privateers they use now. Kuma is a proof of concept for them having their cake and eating it too. He’s the prototype, not for Pacifistas, but for something much worse.
Issho said in chapter 905 that Vegapunk’s latest breakthrough makes the Seven Warlords obsolete. Not the Pacifistas, not Navy battleships or Admirals or anything else, the Warlords specifically. My theory is that Vegapunk plans to use technology similar to the Pacifistas’ to resurrect the preserved moon kings as Kuma-style monsters. Seven kings, seven Warlords. Maybe this was their endgame from the start, and Issho just wasn’t privy to it. Seven new warriors with the durability and obedience of the Pacifistas, but that keep the powers they had in life (like Kuma presumably must have, to be allowed to be distinct from a Pacifista.)
That’s all the relevant connecting of dots and theorising I have for you today. Some of the stuff here makes pretty huge leaps of logic and extrapolates hard, and some later bits are theories based on theories, but the rabbit hole of lore fragments that connect to the room under Marie Geoise runs deep, and you’re welcome to draw different conclusions and imagine different possible solutions for the mysteries here. I'm confident in the story elements I've connected here, just not entirely in the way I've connected them.
Thank you for reading, and I look forward to being wrong about 90% of everything here.
I reckon it's probably right that most of the tech that seems weirdly advanced for OP's setting is probably derived from stuff left behind by the more advanced moonfolk.
Just like Sniper Island, it lives within you heart. 
