Kaymaklı underground city, Turkey.
Built for the Japanese. :-)
<a href="//commons.wikimedia.org/wiki/User:%D0%A1%D0%B0%D1%80%D0%B0%D0%BF%D1%83%D0%BB%D0%BE%D0%B2" title="User:Сарапулов">Сарапулов</a>
<span class="int-own-work" lang="en">Own work</span>