Sealed Tunnels

When I talked about rolling for the condition of an underworld tunnel or area, I said that a Very Bad result for a tunnel indicated a collapsed tunnel. But actually, there are a number of different ways that a tunnel might be sealed off, mostly depending on factors specific to the environment or your general concept:

• Flooding: Tunnel dips down and water or some other liquid has sealed it off. Doesn't block amphibious creatures, so a completely sealed-off section might add amphibians to the vermin typical of such areas.
• Poison Gas: Not physically impassable, but anything that requires air will probably die attempting to pass. As a consequence, much more impassable than Flooding.
• Ice: Flooded, then frozen. Actually pretty impassable until cleared, but clearing it will probably require protection from the cold.
• Mud Flow: Works like Flooding, but amphibians aren't permitted. Creatures that can hold their breath (or don't need to breathe) could pass, however, as long as the mud is still soft. If the mud has hardened, it's pretty much the same as a collapsed tunnel.
• Lava Flow: Much more extreme than Flooding. Only the exotic, enchanted creatures that can swim in lava can pass through such a tunnel.
• Chasm: Quake other catastrophe has left a wide  (50+ feet) gap in the tunnel, preventing access; there may have been a bridge that is now collapsed. Much less isolating than Flooding; any creature able to fly or crawl on the ceiling can pass back and forth. There may even be enough flying animal traffic to guarantee a food supply for an isolated, primitive humanoid tribe.
• Lake/River: Like Flooding and Chasm combined. Instead of a completely submerged tunnel, there's a wide body of water (probably stocked with fish or aquatic beasts.)  There's usually something dangerous enough to make swimming out of the question. A boat or raft may help, as long as there isn't a vertical drop-off on one or both sides.
• Tar Pit: Like a Lake, but a boat is out of the question. Without special measures, anyone attempting to cross tar will probably be stuck. If the tar is boiling, there will also be damage involved.
• Walled Off: Someone deliberately sealed the passage. This will probably be harder to clear than a standard collapsed tunnel, and there's almost certainly something to worry about on one side or the other.