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The men who made it were in fact professional miners and engineers from what I understand.

I can't help wondering about your word "were" (past tense)?

Doing secret work for a ruthless criminal gang must be risky.

Were. Still are. But they were, too.

Sorry for the convenience.

You seem to be right. This article talks about mass graves of diggers being found: https://www.newyorker.com/magazine/2015/08/03/underworld-mon...