Pressure-pump concrete in till it flows out the top. Then, in 1,000 years, pompeii-like archeological gold mine!
Win-win.
heheh...sounds good to me....but that would take a lot of $$$$$
Pump seawater in them. Any building in Gaza that has seawater running out of it should then be bombed immediately and the tunnels destroyed later. Depending on the quality of the cement, seawater will weaken it over time. The tunnels would then collapse.