The Salt Mines at Hallein
This is a great, off-the-beaten-track place to visit.
I believe it has been continuously operated since
the Roman Empire period or even earlier

It was strange to discover that when we entered the mine, we
were in Austria.  After walking along for some time, we came
upon this sign.  Evidently, the
mine crosses over into Germany. At least there was no Duane Toll I put the white line on the photo
to show where the border was.