The Unknown refers to the blank areas on the borders of Gielinor which are inaccessible and represent unexplored territory. Unknown space completely surrounds the World Map and essentially serves as the edge of the game world. Examples of the unknown include the southern border of Sophanem and the western reaches of the Frozen Waste Plateau. The unknown can also be seen around the borders of a Player-owned house. It is coloured black, both on the world map and in-game.
The unknown border south of Nardah shown in-game (left), and on the world map (right).