@Audity:
How do you get from East Blue to West Blue and North Blue to South Blue, and [anything] Blue to [anything] Blue? They never ever intermingle with each other? Sanji was born in North Blue, so how did he get to East Blue?
Unless passenger ships routinely sail the Grand Line to move from one Blue to the other, I think that there are probably at least a few canals or natural channels between the Blues. East Blue and North Blue are effectively cut off from the other two, but shipping is probably possible between the two.
Of course, traffic between the North and South hemispheres probably goes through Mariejoa.
I didn't know that's where Mariejoa was. Is there a reverse mountain there, too?
That's the most likely location based on the fact that:
1. Oda specified early on that there was a city based on the Red Line (well, two actually, but I suspect Roguetown was originally intended to be there instead of in East Blue; one is a retcon).
2. The King of Lvneel had to ask permission from Mariejoa to enter the Grand Line, which suggests that they control one of the entrances to the Grand Line. A centralized location and ready access to the four Blues would allow them to dominate world politics much more efficiently; if Mariejoa lacks sea access, then it's an awful location for the headquarters of a global agency in a world dominated by water.
If there was, North and West Blue could skip half the Grand Line, so probably not.
Again, the World Government may not care who leaves the Grand Line, but they certainly care about who enters it. The various forces of the World Government and Naval Headquarters would make the entrance at Mariejoa even more dangerous than Reverse Mountain.
Besides, if Reverse Mountain is the only entrance, then the World Government could easily choke off the entrance of new pirates from the Blues by basing a fleet at the Twin Capes, which could also operate in that entire area of the Grand Line.
Also, it seems very easy to me for really powerful Devil Fruit users to get past the Calm Belt, and go straight to Raftel.
If it were that easy to get Raftel, then you could simply sail on the Grand Line until you came into sight of the Red Line, which would make ordinary navigation possible again.
Raftel has to be in some exotic location that cannot be reached by ordinary means.
@Xetalimn_Erket:
As such, is it possible that the Calm Belts continue into Grand Line waters after passing through Red Line?
The Calm Belts most likely function as Tropics (which are also regions prone to being becalmed) and run parallel to the Grand Line.
- While we can define Grand Line as existing solely between Red Line, this does not explain where exactly Raftel is; it's allegedly at the "end of Grand Line", but Grand Line looks to end at Marieoja, not Raftel.
No, keep in mind that the Grand Line is basically the Odaverse Equator. It stretches completely around the world and intersects the Red Line at two points; one of which is probably a water way while one is mostly land.
- How would one actually sail into Grand Line waters without crossing the the Calm Belts if they have no definite start/end point (aside from proximity to Red Line).
Each Calm Belt is probably broken up at six locations; twice in each Blue by the major currents that run through them and the Red Line itself. At a guess, the main current of the Grand Line diverges into five seperate currents near the Red Line with one heading off into each Blue and one continuing on.
The Calm Belts more or less start at a Red Line, head east-west (or west-east if you prefer) until they reach the current of that respective Blue. That current cuts across them and allows sailing access to the entrances and exit to the Grand Line. After the current, the Calm Belt picks up again and crosses the Blue until it reaches the next section of the Blue Current. After that interruption, it runs into the Red Line.