In early April 1943, an Allied agent reported "enormous trenches" being excavated at the Watten site, and on 16 May 1943 an RAF reconnaissance mission led to Allied photographic interpreters noticing unidentified activity there. Other large facilities were observed to be under construction elsewhere in the Pas-de-Calais. The purpose of the construction works was very unclear; Lord Cherwell, Winston Churchill's scientific adviser, admitted that he had little idea what "these very large structures similar to gun emplacements" were but he believed that "if it is worth the enemy's while to go to all the trouble of building them it would seem worth ours to destroy them."