Encore Puzzler: Trees and Mines

RAY: Some years ago, my wife and I took a trip south, down the east coast, from our fair city of Boston. I forget what our destination was.

TOM: But it was all down hill.

RAY: Right. I forget what our final destination was, but somewhere around West Virginia I got the notion that I wanted to explore some abandoned mines. I engaged a guide named 'Lefty...'

TOM: Formerly an Everglades tour guide.

RAY: Right, but since that fateful swim with a 'gator, he could no longer operate the controls of the tour boats. So he takes me through a few. Now, my only recollection of mines was from the old westerns, where the roof and walls were shored up with timbers.

TOM: Like Volvo does.

RAY: I was surprised to find that in fact they still do that. I asked Lefty what kind of wood they used, because I didn't recognize it. He said "I only know 'gators, not trees." To my surprise, all the other mines used the same wood. I would have expected them to use Oak or Ash, strong woods plentiful in the area. But it turned out to be Tulip Poplar, which I thought couldn't possibly be near as strong as the aforementioned trees.

Question: Why was this the only kind of wood used to shore up these mines? Hint: It had nothing to do with the trees' proximity to the mines.

The reason has to do with the sound certain woods make when they're nearing their break point. Many stronger woods would give no noise before the point of giving out -- cracking and breaking at the same instant. But Poplar will begin to break at the point it begins to strain or stretch, making loud cracking noises. When this would happen, you'd hear the following noises if you listened carefully: "Get out of here!!"

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