This category has to do with immersive, graphical, persistent state environment, virtual worlds — like a Wide Open Sandbox MMORPG that is all sandbox. These worlds could someday become The Metaverse, while some argue that they already are. Their counterpart, Online Games, may use the same Physics Engine and physical hardware, but are centered on a particular game title, and unlike the often long-lived Virtual Worlds, are likely to become an Orphaned Series. The line between Online Games and Virtual Worlds is blurry, as extensive modification can turn one into the other; such as a private server running World of Warcraft without mobs of monsters, or a Second Life island that features a user-scripted paintball game that is only playable in a limited area. For in-universe virtual worlds, see Lotus-Eater Machine, Cyberspace and Inside a Computer System. Compare Immersive Sim.