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.)
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.)