My favorite death trap is probably Facit Sandstone Mine, it's a complete labyrinth of passages inside the pillar and stall mine. I have seen talk of 120km of passages, this was quoted after the rescue of 3 children who were missing in here for a few days. Navigating your way through is interesting as it all looks the same and is sort of laid out in a grid pattern.
Apart from that it's one of the most unstable places I've seen and that's throughout, the floor often mirrors the shape of the roof as the rock falls off, people have used string in the past to mark the way through but this is now buried under fresh rock falls. I've not encountered bad air in there myself, but a couple of people I know have done in the past.