There was another thread about that somewhere. The general consensus was that you simply can't reach that area.
Probably a dead end, created to give players the impression that the temple of the dead isn't just a couple of walkways and an old lab, but that it might be very large.
Remember from the Kickstarter campaign when part of the stretch goals was a 10 level dungeon under the Phantom Forest that never ended up happening and now we just have to sort of hope Larian will still deliver it at some point because the game has been sucessful and dominated sales charts for weeks? This seems to be what's left of that. At some point it really was supposed to be something awesome.
