From Elantris:
> "Over there," Karata said, pointing at the far end of the rectangular room, where the wall held only a single mural. It depicted a large mirrorlike blue oval. An Elantrian stood facing the oval, his arms outstretched and his eyes closed. He appeared to be flying toward the blue disk. The rest of the wall was black, though there was a large white sphere on the other side of the oval.
This would be the hint. The white sphere probably represents another world, or some sort of nexus, or whatever.