You try and open the lock and after a bit of a struggle the rusted mechanisms swings open. Looks like the crab did it's job. Gingerly you push to door open and step out into a dusty corridor, badly deteriorated from years of disuse. Rotted wooden beams in the ceiling creak at your unexpected arrival and you smell the musty scent of age and decay.
You look to the left to find the corridor goes one about 30 meters before turning a corner right, along the walls are dotted torches though only a few remain lit. You wonder why someone would bother lighting torches and yet allow the corridor to reach such a state of disrepair. In the dust, you see a trail of footprints from the corridor lead up to your door. There seems to be no other doors this side of the corridor, nor windows. There is however a small grill set in the floor about halfway between you and the corner, you're not sure, but you think you can hear running water echoing from below.
To the right, you notice the corridor goes on for about 10 meters before succumbing to a cave in. Huge boulders block your path and your ability to see any further. However, the cave in has presented another opportunity - there seems to be natural light shining through the hole in the ceiling, perhaps you could climb your way out up above the rubble.
You look to the crab for guidance, but it just stares back blankly at you, shrugs and then appears to go to sleep.