You head right and start down the walkway leading downwards. The path curved along with the tunnel and rats dart ahead of you in the dim green light. After some time the walkway ends. To your left is a bridge leading to the other side but the center has collapsed and the remnants look unstable. To your right is a grimy solid metal door. You can knock on the door, try to open it, or head back and follow the walkway leading upwards.