The Arch is very mysterious and uninhabitable, it is less of an Island, and more of a rock formation.
This is one of the only 2 locations in the game where sulfur can be found, it also has Mithril. It is very hard to traverse and requires either large amounts of Ladders or very good placement of random (preferable welded) items. It is very far away from all of the islands, The Mainlands being the furthest. Unlike the mainland mithril cave, it has two veins of mithril, one being directly on top of the arch, while the other is in a difficult to reach mini "overhang" style cave, which must be reached with the use of welded items or use the pathway going to
the ore. Interestingly, it also has coal on it, making it have all the ingredients for gunpowder. If your boat just happens to burn down while you're here and you have no teammate in the server, you're pretty much doomed to die here, since no edible resources can grow here. That being said, it's smart to carry around a spare boat, since mithril campers like to stay here and can possibly strand you there.