RealClearDefence has a fine idea to solve Israel's Hamas tunnels problem - pump in sea water. The Gaza aquifer is already too polluted to be potable. Everywhere in Gaza is close to the sea. And the best part is that there is no need to fill them completely, a tunnel with 2-3 feet of water is essentially unusable.
The engineering is straightforward. Egypt flooded thirty-seven cross-border tunnels in southern Gaza back in 2015 in what stands as a practical proof of concept in this location. Seawater from the Mediterranean would be pumped directly into the tunnel openings through short pipelines.
In the short term, think of flooding Gaza’s tunnels as humanitarian assistance. By eliminating the need to keep bombing them, flooding would reduce civilian casualties and other collateral damage. In the long term, think of denying Hamas access to the tunnels as an A2AD stratagem.
If tunnels are below sea level, perhaps a syphon would work?