Katwijk aan Zee, Netherlands. A strange cloud (probably a sort of a cumulonimbus cloud) arrived the shore in a few minutes time.
<a href="//commons.wikimedia.org/wiki/User:Mmparedes" title="User:Mmparedes">Mmparedes</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>