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.
Answer:
✔The first thing
✔Then
Explanation:
The above words are the details in the text that indicate a chronological structure.
Chronological order/structure refers to a way of organization in which events are arranged or presented in order as they occur or occurred in time. It's known to be a structure of sequence.
Words that denote chronological order/structure in texts are first, third, second, until, then, after, later, at last, next, etc.
In the text, the use of "The first thing" and "then" actually indicate a chronological order.
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