Israel builds wall deep underground to thwart Hamas tunnels | World news | The Guardian
https://www.theguardian.com/world/2016/ ... an-tunnels

'We're gonna make Hamas pay for it... it's gonna be so great, folks!'

Surely they'll just dig their tunnels deeper.

It depends how deep deep is. If they are down to bedrock, then it isn't practical. You can dig a tunnel in sand fairly quickly and quietly, without much noise. If you are having to cut it out of rock, you need heavy equipment, somewhere to hide the rocks, a lot more time and due to the noise, you will be much easier to hear coming...