During periods of fine weather in October 1916, British reconnaissance flights had reported new defences being built far behind the Somme front; on 9 November, reconnaissance aircraft found a new line of defences from Bourlon Wood to Quéant, Bullecourt, the river Sensée and Héninel, to the German third line near Arras. Next day, an escaped Russian prisoner of war, reported that were working on concrete dug-outs near St Quentin. Behind the Fifth and Fourth army fronts, the course of the Hindenburg Line was further away and the winter weather was exceptionally bad, which grounded aircraft and made air observation unreliable. On 11 December, a reconnaissance in the area of Marcoing reported nothing unusual, despite flying over the new diggings. German fighter opposition in the area became much worse, with more aircraft and the arrival in service of superior aircraft types in the late summer of 1916. Three intermediate defensive lines begun in late 1916, much closer to the Somme front, were observed by British reconnaissance aircraft, which made fragmentary reports of digging further back unexceptional.