Once your game world size crosses a specific threshold, you need to move the world instead of the player because numerical accuracy starts to become troublesome

Once your game world size crosses a specific threshold, you need to move the world instead of the player because numerical accuracy starts to become troublesome (most engine code runs with floats, which gives about 6 decimal digits of precision). The first engine that I am aware of that did that was that of Dungeon Siege. There's an excellent post mortem around about the challenges of this approach.