Around the Senlan Islands is this reef, which seems to be very relative on it's location. It is covered in a deep mist and fog and it goes on way before the Senlan Islands can be seen, which is why many people get lost here. Only people of the Islands know how to navigate through the mist and not crash into the Cliff of Senlan, like most do.
All the way to the north of these islands is the Polar Ocean where many travel if they want to get an out of the way route or if they just want to see. It's covered in ice and there are many iceburgs here, but you could route your way through. A famous ship crash of the "Titan" ship happened here and the remains now are a landmark as the broken ship is now remaining on an iceburg.