In a tale that sounds like a cross between a disaster movie and an episode of 'Extreme Engineering,' a tunnelling expert has revealed how rescuers managed to pull two men alive from a blocked Nepalese hydropower tunnel. Arnold Dix, president of the International Tunnelling and Underground Space Association, has been advising Nepal's hydropower authorities on how to dig through the blocked tunnel to Trishuli 3A Hydropower Station, one of several stations hit by last week's massive flash flood on the Nepal-Tibet border.

'It's like layer upon layer of layer of miracles' that they found Sanjaya Shah and Kabir Maharjan on Friday, Dix told the BBC, given the multiple challenges faced. The first hurdle? Simply locating the station in a landscape rendered nearly unrecognisable by the flood that has so far killed 1,300 people. 'I've never known a rescue operation where you had to look for an entrance of a tunnel that was previously up the hill, that was now underground,' Dix said. The avalanche completely buried the entrance, so his team triangulated its likely position using engineering techniques, old topographic maps, satellite imagery, helicopter footage, and 'anything else we could find on the surface of the mountain such as power poles.'

They found it, but then came the next challenge: locating survivors. 'We knew they would be up ahead, but we didn't know where exactly. So you just have to start digging in the right direction. And you listen out if you can hear anybody,' he explained. Rescuers had to clear a tunnel entirely blocked by mud and rocks - some bigger than cars - and used explosives to clear the way, something they would 'normally never do because of the risk of blowing up people [trapped in the tunnel] and blowing ourselves up.' Dix credited the Nepalese army for doing 'a wonderful job' in managing the explosions with precision.

Meanwhile, they discovered the tunnel was still full of water. Using excavators and pumping machines, Nepalese rescuers dug trenches from the tunnel to the river outside to drain it. Finally, they dug a 'tiny tunnel the size of a person' through the debris at the top of the tunnel, using the original tunnel's roof as their own - a clever move that also mitigated flooding risk.

In the early hours of Friday morning, rescuers heard what they thought could be voices. According to local reports, they called out 'don't worry, we're here to rescue you' and heard back a faint 'okay.' Shortly after, they pulled out Shah and Maharjan, alive.

But with dozens still thought to be trapped, the work is far from over. Dix estimated they had dug about 60m from the entrance and need to dig another 150m to reach the station deep inside the mountain. Rescuers hope debris didn't reach the station - if it did, 'there would be all kinds of complications.' The team still faces dangers: 'Up ahead, even now, we might run into water which would drown the people tunnelling,' Dix said. 'And we also have the risk of another flood outside happening, so water could come from behind. So there's the risk of facing water on both ends.'

But Friday's success has buoyed spirits. 'It's a big relief, that we are on the right track. And it gives us renewed energy that our thinking [on how to conduct the rescue] works... we have never done something like this before.' Dix said they are now heading toward the 'Goldilocks zone' - the uppermost level of the station where there's space and conditions 'more friendly to life.'

'We got two people out. If we were playing at a casino, some might say it's time to walk away, but we say game on, time to take on the house,' Dix said, channelling his inner high roller. Let's hope the house doesn't win this time.