Marineford, the location of Marine Headquarters.
Although it was also on the Grand Line, ordinary Log Poses mysteriously could not record its magnetism. Whether some peculiar technology was responsible, or the island's own characteristics were why Marine Headquarters had been established here, he did not know.
To reach Marineford, aside from using an Eternal Pose, the only option was to follow the peculiar current connecting it to Impel Down and pass through a massive Gate of Justice.
Honestly, with those two geographical factors coinciding at Marineford, Xie Wen felt it was more likely that the island had been moved here from somewhere else, then built up and modified artificially.
Another piece of supporting evidence was that, viewed from above, the entire island of Marineford formed a perfectly symmetrical crescent. However wondrous nature's handiwork might be, it could not possibly produce an island like this... probably.
This was the One Piece world, after all, and there were far too many strange geographical features, so Xie Wen did not dare jump to conclusions.
But whether Marineford had been built artificially or not, the Marines had now turned it into an enormous military fortress. They had erected towering walls along the island's perimeter, with large numbers of Marine warships guarding its surroundings, leaving only the opening of the crescent as an entrance and exit.
And having read the manga, Xie Wen also knew that this sole entrance held a trick of its own. If an enemy broke through the warships guarding the perimeter and charged inside, walls bristling with cannons would rise along Marineford's inner edge, giving the invaders a little taste of overwhelming firepower...
Well, it really was just a little taste, because in the manga, that move did not seem to have done much damage to the Whitebeard Pirates. In the