These were not the first mines in the Ypres Salient. From 1915, British engineers started digging, but it was not until the winter of 1916 when they were joined by Canadian, Australian and New Zealand engineers, that mass tunnelling started under the German trenches. At St Eloi, six mines were detonated on 27 March 1916. At Hill 60, a gallery was started in summer 1915 and known as "Berlin Tunnel". The 1st Australian Tunnelling Company took over mining operations at Hill 60 in November 1916, led in part by Captain Oliver Woodward. Twenty-six deep mines were eventually dug in preparation of the Battle of Messines in 1917, some up to long and up to deep, altogether containing 600 tonnes of ammonal explosive. To solve the problem of wet soil, the tunnels were dug in the layer of "blue clay", 80–120 feet (25–30 m) below the surface. The galleries dug in order to lay these mines eventually totalled over in length, and had been constructed in the face of tenacious German counter-mining. On several occasions, German tunnellers were within yards of large British mine "chambers." The mine at Petite Douve Farm was discovered by the Germans on 24 August 1916 and destroyed by a counter-mine.