A tunnel at Down Street, previously partitioned into offices (left) and a corridor (right). The floor was raised to give the offices a flat base, and the corridor kept as narrow as possible.
<a href="//commons.wikimedia.org/wiki/User:Andrew_Gray" title="User:Andrew Gray">Andrew Gray</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>