An environmental activist who spent 12 days in a tunnel network underneath Euston Square Gardens in central London says the protest is likely to be the first of a new wave against the climate emergency using subterranean tactics. Speaking exclusively to the Guardian in his first interview since leaving the tunnel network on 6 February, Lachlan Sandford, 20, known as Lazer, said the protest to raise awareness about the environmental destruction that activists believe the high-speed rail link HS2 will cause would not be a one-off. He highlighted the construction of a second, smaller tunnel at Highbury Corner, also in central London, from where protesters were evicted after a few hours. “As you can see from the recent Highbury Corner eviction, this tunnel is just a start,” said Sandford. “There are countless people I know who will do what it takes to stop HS2.” Phase 2a of HS2 connecting the West Midlands and Crewe received royal assent in parliament on 11 February. Sandford’s actions in the tunnel hit the headlines after he was locked to a complex steel and concrete device, which activists underground referred to as “a concrete cake”. It consisted of a metal safe encased in concrete, with more concrete inside, and Sandford locked to an “arm tube”. He also had a lock on his ankle. “It was an incredibly uncomfortable and painful thing to endure,” he said. “But nothing like the pain and discomfort that the climate and ecological emergency is bringing and other people around the world are already feeling.” There are seven of the original nine activists left in the tunnel, including Sandford’s sister Blue Sandford, 18, Dan Hooper, known as Swampy, 47, and Swampy’s 16-year-old son, Rory. The plan was for Sandford to be the first to leave, but a 17-year-old activist left the tunnel the evening before because she was feeling unwell. “That was my job from the start, to be the person holding the down shaft and protecting the entrance to the tunnels at the bottom. We were thinking it would only take them three days to get to me but actually it took them 11,” he said. As supporters followed news updates on the lengthy process of removing Sandford from the “lock on” – he was locked on for 30 hours and it took the bailiffs 25 hours to extract him from it – they expected the bailiffs to bring him up from the tunnel. However, he grabbed the lock-on after he was released and ran back into the tunnel, evading capture by the bailiffs. He said the decision to do that was not premeditated. “That was spur of the moment,” he said. “I escaped back into the tunnels for a rest. I was pleased I got a chance to say goodbye to Blue and the others properly.” He said he negotiated lights and other essentials for the tunnellers before leaving the tunnel voluntarily. HS2 says it provided lights but not other essentials. The time Sandford spent in the tunnel is far from his first brush with activism. His father, Roc Sandford, is a Scottish laird who lives off grid on the small Hebridean island of Gometra, pursuing a carbon-neutral way of life. He has also been active in XR Youth. “With my sister Blue and others, we chained ourselves across the entrance to a fracking conference a couple of years ago,” said Sandford. “I think it was hard for Blue to lose me from the tunnel community. I love them all dearly and I’m looking forward to when I see them again. “The HS2 bailiffs are hardcore. They are being fairly heavy with us and it’s pretty extreme, but nothing like what is coming if we don’t get our voices heard. People of my age are not being given a choice because the government isn’t doing what’s needed and that’s when I started protesting more. We need to figure out how to get more MPs to back the climate and ecological emergency bill, there’s about 100 so far, so get on to your MP, and get them to scrap HS2 as well.” Roc Sandford said of his two children involved in the tunnel protest: “I’m so proud of them, they are standing up for what they believe in. But they shouldn’t have to, it’s too much for young shoulders like theirs to carry. We all need to help them.” One of the first things Sandford did after leaving the tunnel was get some falafel from his favourite place in west London. But the rigours of life underground have not deterred him from environmental activism. “I can’t divulge any of my future plans for tactical reasons, but I’m nowhere near finished with protesting,” he said.