I think it all depends on how much air is flowing through the cave or tunnel (either naturally or with fans) and how many people are in a confined space breathing at any moment. With mining or tunneling you can also hit pockets of methane or other gasses that can be explosive or displace oxygen, but I think that's mostly a hazard in coal mines.
I'm pretty sure this isn't true, but I'm not a dirt scientist. You might get radon gas seeping through the earth though.
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I think it all depends on how much air is flowing through the cave or tunnel (either naturally or with fans) and how many people are in a confined space breathing at any moment. With mining or tunneling you can also hit pockets of methane or other gasses that can be explosive or displace oxygen, but I think that's mostly a hazard in coal mines.
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