We know from Anime and Manga sources that the name of the area inside the Calm Belt is named 'Grand Line' and it's supposed to be the path to follow if we want to find Laugh Tale.
Now, what if I told you that the name of the Grand Line itself is telling us more than expected?
Remember how the World Government 'confused' (or hid) Gol D. Roger's name
The pronunctiation of Gold Roger in Japan is as follows: Gōrudo rojā
So it's easy to pronounce Gold Roger, but in fact it is, as we all know: Gol D. Roger
So, split into syllables: Gōru(Gol) do(D.) Rojā(Roger)
or correctly: Gōru D. Rojā
Now, applying this to 'Grand Line': Gurando Rain.
So, split into syllables: Guran(Gran) do (D.) Rain (Line)
or correctly: Gran D. Line 
WAIT! It is not over yet:
Gran D. Line would make sense since 'Gran' is the word for 'Grand' in Spanish, and we know that Oda likes to use some Spanish names in his series, but if you're not convinced, read below:
There is a map that you can see for yourself in the internet that was drawn by Eiichiro Oda himself! (Search for Oda Sketch Map)
And guess what? He wrote 'Gland Line' instead of 'Grand Line' in his map.
Coincidentally, 'Grand Line' and 'Gland Line' are both pronounced the same way in Japan: Gurando Rain.
So if we will write it like Oda with the L we are left with the next name: Glan D. Line (Guran D. Rain)
K and G also sound very similar in japanese, so we could hear (Kuran D. Rain) as well, resulting in Clan D. Line.
Coincidentally, all the Road Phoneglyphs and Ancient Weapons are inside this line.
The world government changed the names of Gol D. Roger and Hito-Hito no Mi to hide them, so why not even more things? After all, there is a whole century of history unregistered and they could easily alter it.
The reason why they might have changed it to 'Grand Line' is simply because 'Gland Line' would sound gross to the readers 
also, it would not make any sense for this big line across the world.
Now, I´m basing the next point in my theory in the fact that Wano country was surrounded by walls in an 'unknown, artificial way' according to the previous Shogun. The walls hide Pluton inside and Zunesha is waiting to release it for JoyBoy.
So my best bet is that JoyBoy (or some friend of his) was somehow capable of creating enormous walls (not so hard to guess, I know). In One Piece there is at least one confirmed god, and there is a hint of a second God in SkyPea (they have a statue dedicated for him due to the fertile land) and god fruits can do absurd things through inimaginable powers.
We know that there is a power capable of erasing whole islands at will... so why would it not be possible to create walls?
My theory is that the World Government wanted to destroy the Clan D. and the Calm Belt was sepparating the 'Clan D.' from the rest of the world. They had the help of the Kyojin and an amazing ship called Pluton to fight, plus the powers of Nika.
They had a 100-year long battle where the Clan D. lost to an unexpected power from the World Gov. and decided to leave hints behind and seal the powers that they had to avoid the World Government to get them before the future generations had the chance of recovering and fighting back. The World Goverment erased these registers from history to make the world forget about the rebellion.
That is why the World Government do not want to let go of the strategic position in the entrance of the Clan D. Line. If we look at the map, the 'Red Line' makes it impossible to escape from the Calm Belt after you've passed Mariejois without special ships since the upstream rivers are not acessible from there. This is a great metaphor for the 'Red Line' (the Clan D. must NOT cross the Red Line).
The Kyojin were allies of the Clan D. during the war, allowing them to cross under Mariejois freely, and for this reason they were sold into slavery and rejected as a race after Clan D. lost the battle.
After the war with Shirohige where the base of the Marine got destroyed, they decided to set up AGAIN in the entrance of the Clan D. Line. There´s something really important about defending this location 
These facts would serve to explain many things:
- Road Phoneglyphs: They are all located inside the Clan D. Line, right after passing Mariejois
- Pluton: It is buried under water in Wano, with a wall made in an unknown way.
And the world government wanted to find the blueprints, while the ship itself is hidden from the world waiting for JoyBoy, as Zunesha and Momonosuke correctly hinted - Every ancestral weapon is in the hands of old Clan D. allies: Kyojin (Poseidon), Wano (Pluton)... Just as the Road Phoneglyphs: Zoan, Wano, Kyojin Kingdom... and Big Mom stole one of the phoneglyphs from an unknown location
Extra: If you take the World Government's symbol and put it just in the middle of the Red Line, you will see how it is making a perfect match with the mountain and the crossed rivers. Maybe another hint that they are controlling the Red Line? Why those 5 circles in that position?
I hope you liked my theory and found some fun in reading it 
