It is on Stadiou street and Aiolou street. The Stadiou street was a torrent which collected the rain water from the Lycabetus hill and nearby area. It was covered in mid 19th century. However the street below is hollow.
<a href="//commons.wikimedia.org/wiki/User:Athenswalk" title="User:Athenswalk">athenswalk</a>
<span class="int-own-work" lang="en">Own work</span>