[Radek]

I think the "tunnel" behind the oracles is a result of simplifications in the rendering algorithm. You can see such false "tunnels" sometimes in the game. The "tunnels" appear when you look from a rather strictly specified point in a rather strictly defined direction. In the other words, they do not do any harm.
Everytime you "see through" only from a specified location and only if you look in a certain direction, then consider it an artefact and ignore it. AFAIK, there are no "hidden passages" (passages that pretend to be walls, a forests, and so on but that are passages in fact) in the game.
The "tunnel" behind the oracles might be such artefact. I have tried to see the place where the oracles are. I cast Lavawalk and tried to go through the fires. The fires pushed me back no matter what I did. (The fires didn't burn me - the Lavawalk worked.) The Vanish spell fizzled. I think there is no normal way of entering the place with the oracles.