Small pool on the observation deck of the Capbreton blockhaus #5 (Landes, France). There is a pool like this one at the 4 corners of the observation deck (roof supports ?).
<a href="//commons.wikimedia.org/wiki/User:Tangopaso" title="User:Tangopaso">Tangopaso</a>
<span class="int-own-work">Self-photographed</span>