The danger zone in food safety terms refers to the temperatures within which food should not be kept or else it will probably become contaminated. Bacteria can easily develop in food if it's kept within the temperatures between 41 and 140 degrees Fahrenheit for more than two hours. In such cases, food should be thrown away. This is the reason why fridges should be set at a temperature of under 4 degrees Celsius anyway to prevent food from going bad.
I don't know much about the Sydney mines, but from the context, I can infer that the tunnels were unsafe. Whether being that they were collapsing, or they were filled with poisonous gasses.
