As far as we can tell for now, the way back is a mystery. This world has a mind of its own. It pulls people here in a bid to save itself. That's the most crediable theory at present.

...Actually it's getting closer to four years for me. I consider this world my home for however long I'm going to be here. [He gestures to the house about them. Books, scrolls, and maps litter the work desk he rests on the edge of.]