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The "softly, softly" approach to drilling escape holes, and gauging the auger's impact on the already fragile and "still moving" mountainside, were key to freeing the 41 men trapped beneath the collapsed Silkyara tunnel in Uttarakhand, tunnelling expert Arnold Dix told NDTV Wednesday, hours after a precise 17-day rescue operation came to a successful conclusion. Arnold Dix, a tunnelling expert and a professor of engineering, also told NDTV he was not surprised by 'rat hole' miners - practitioners of a mining process banned by the Supreme Court in 2014 for being unsafe and causing environmental pollution - being the key in the Uttarakhand tunnel rescue. "My view was that we needed to go softly... I was confident nobody would be injured as long as we took our time and were careful. So we had to go 'softly, softly' and, in the end, we achieved our mission 100 mm at a time... digging by hand," Professor Dix explained, "This was important because we didn't want to disturb the mountain and cause another avalanche or disturbance." "I did anticipate 'rat hole' mining would be the breakthrough. That was part of the advice I gave because I could see that with every big machine used the mountain's reaction was more severe." READ | The Crucial Role Of Outlawed Rat-Hole Mining In Tunnel Rescue In the days that immediately followed the tunnel collapse, rescue teams used large earth drilling machines, or augers. These, however, would prove to be unsafe because the vibrations produced while it was drilling raised fears of a landslide; on one occasion drilling was halted because of a 'cracking' sound from the collapsed section, which raised fears a cave-in could bury the workers. The larger drills also broke down repeatedly after hitting metal obstacles, specifically steel rods from the collapsed tunnel structure. This forced more pauses while laser cutters were deployed. READ | Tunnel Rescue Op To Take "12-14 Hours More" After 2nd Metal Obstruction