During the Second World War, a 6.3 km long tunnel system as an air-raid shelter for up to 50,000 people was driven into the Schloßberg of Graz by forced laborers.
<a href="//commons.wikimedia.org/wiki/User:Isiwal" title="User:Isiwal">Isiwal</a>
<span class="int-own-work" lang="en">Own work</span>