So far I've only found the initial one that you explore with Sam. I've found other access points , it seems like, but they're all locked and I can't get inside of them... and I've completed explored the tunnels and there's nothing but dead ends. I see a brick wall, but I don't know how to break it. There aren't any powder kegs lying around, and there's a door, but I have to unlock it from the other side. So.. my question is, how do I access the other parts of the underground? Is it story-related? I'm at the beginning of sequence 7.
You should unlock all of them in the tunnels. If you find a locked room, there's generally a way to get around it in that room. Just look around, and don't be afraid to put down your lantern in front of the locked doors so you can climb on things.
Gamertag is Madigari, for both Xbox 360 and PSN. Of all the things I've lost, I miss my originality the most.