It defies belief that the Israelis didn’t emplace lines of sensitive tiny microphones underground, all along the Gaza/Egyptian border, years ago. Continuous monitoring, triangulating and filtering would have located all the tunnels in real time as they were dug. This is, I admit, the seismograph technique noted above, but it’s passive and doesn’t need any exterior pings or explosions. They wouldn’t have to be out in the open, any basement would do.