The Millenium Blue lies somewhere between the Red Line and the New World. Sometimes, luckless pirates who want to reach the New world end up here. The Millenium Blue is a highly miraculous place. Its outskirts consist of lagoons with underwater caves. The pirate Jiguro and his Parable Pirates specialize in using submarines to reside in these caves. Next to the center of the Blue, the water becomes thick and so soggy that no normal ship would be able to drive through it (The Thousan Sunny was, thanks to its special engines). One can even walk on that water. In the center of the Blue rises the Arbalessia fortress, a giant tower where the Special Deletion Unit and (sometimes) Admiral Kizaru reside. The fortress is surrounded by a giant mangrove wood.
Before the World Government made it its training grounds, it was inhabited by a tribe of strong warriors. But in a single battle, all of them were killed, and the Government started to rule here. Usually, there is at least one Marine Admiral residing in Arbalessia, and at least fifthy thousand World Government soldiers, of whom only two thousand guard Arbalessia.