Blowing up the world

This last gaming session the group was faced with a dilemma.  We are currently camped outside of an ancient city with a perfect obsidian wall around it that is 40 feet tall.  There are people guarding the city who claim the wall was put there by ancient gods to keep a terrible evil locked away inside.  The people want to recruit us to join their ranks to fight against said evil and other nasty things in the world.  But we just want to find out what exactly lies on the other side of that wall!

In a campaign where you can trust all the NPCs that aren't attacking you the choice would be easy.  We should join the group of paladin types who is trying to protect everyone and take the next step on our quest.  However, we are adventuring in a world where people act much like real people and it is not at all clear to us that the story we are being told is true.  After all, the city has been shut for thousands of years.  Does anybody really know what is inside after all this time?  Is the story we are being fed true at all?  Maybe just beyond those walls lies adventure, treasure, and monsters to be slain for massive gain of experience!

There is the distinct possibility that should we decide to ignore the defenders of the forbidden city and scale those walls that we will end up dead, or being responsible for freeing a terrible darkness into the world.  The first one isn't very interesting but if it happens I will roll with it; you rolls the dice and you takes your chances.  The second possibility, while it would be devastating for my character, sounds pretty fun to me.  So now our mission is expanded from 'establish good trading relations with some folks' to 'save the world from a horrible fate that we brought upon it'?  Sounds like a great adventure to me!

I hope the GM is ready for us to do something colossally stupid.  I know that when I run adventures if the PCs really do something crazy I let it happen and the world simply adjusts - sometimes they decide to resurrect an ancient demigod intent on sacrificing hordes of people to bring back the old gods, and that is fun.  I mean, if the world just dies because a couple of lunatics climb over a wall to see what is there then I guess that world is probably screwed anyway because eventually somebody is going to climb that wall to see what is on the other side.

Time to do something dumb, I think!