A pretty special hiking trail in Austria is the 1. Wiener Wasserleitungsweg which leads along the water supply line from the mountains of Lower Austria down to Vienna. So one can learn about the sources of Vienna’s water just by following a path.
Personally I got in touch with this trail by walking along the Genussmeile, a wining and dining event in the vineyards between the cities Bad Vöslau and Mödling. This mile followed exactly the water supply line. Generally the line is hidden below the path but sometimes one can see maintenance buildings looking like guard houses.