In the island expedition to Un’gol Ruins, there’s a cannon that can spawn on the shore which sends the player to a ship. On the ship, they can fight the rare mob Dreadbeard, and there’s a matching cannon on the ship that takes them back to the shore. The cannon on the shore is marked with a “Portal” icon on the map and minimap.
The ship that the cannon sends the player to is WAY, WAY below the map. Like, thousands of yards out of bounds, in the void. If you point the camera up from there, you can see the entire island far above you in the sky.
I’m not sure if this always happens or just sometimes. The ship was at coordinates (46, 50) on the map for me.