As you head down into the tunnel, the air grows more warm and moist, and the tunnel goes from looking like a natural cave to obviously being worked stone. You occasionally hear faint chittering sounds, and once or twice catch a brief glimpse of cockroaches the size of cats scurrying around corners or into cracks away from you. You also see a couple of humanoid remains, nothing left of them but bones and rags, any valuables obviously looted long ago. The echo of your footsteps ahead lets you know that the tunnel must be opening up into something bigger, just outside of your range of vision.