Why would the defenders allow the tunnels to exist? Demolishing tunnels isnt expensive, if attackers prefer to attack through tunnels there likely isn’t enough incentive for defenders to not demolish tunnels
The expensiveness of demolishing tunnels scales with the density of the tunnel network. (Unless the blast effects of underground explosives are generally stronger than I expect; I haven’t done calculations). For sufficiently dense tunnel networks, demolishing enough of them would actually be quite expensive. E.g. if there are 1000 tunnels that you need to demolish per 1km of frontline, the quantity of explosive needed to do that would probably be greater than the quantity you’d need to make a gigantic minefield on the surface. (Minefields can be penetrated… but also, demolished tunnels can be re-dug.)
Why would the defenders allow the tunnels to exist? Demolishing tunnels isnt expensive, if attackers prefer to attack through tunnels there likely isn’t enough incentive for defenders to not demolish tunnels
The expensiveness of demolishing tunnels scales with the density of the tunnel network. (Unless the blast effects of underground explosives are generally stronger than I expect; I haven’t done calculations). For sufficiently dense tunnel networks, demolishing enough of them would actually be quite expensive. E.g. if there are 1000 tunnels that you need to demolish per 1km of frontline, the quantity of explosive needed to do that would probably be greater than the quantity you’d need to make a gigantic minefield on the surface. (Minefields can be penetrated… but also, demolished tunnels can be re-dug.)