That's Clearly Not Normal Wood

http://www.snopes.com/photos/architecture/nagasaki.asp
Ah, here you go. The first arch was in Nagasaki after the bombing. The second was in an entirely different area that isn't in Nagasaki. Either way, for it to withstand a nuclear blast is amazing.

But still, both structures withstood when everything else was dystroyed. Whatever they are made of must be incredibly strong.
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Edit: You can tell in the photos they are in two different places. Look at the mountain range

You can take pictures from the other sides of things to though buddy