They can not flood the damn thing?
There are probably good technical reasons for not flooding the mine. However, that being said, an alternative might be to pump extremely cold inert gas into the mine. An excellent alternative would be liquid argon, at around -300F.
It is an abundant gas, there is more argon than CO2 in the atmosphere, so the great difficulty would be in cooling it until it becomes very cold gas. The best bet would probably to build a facility for just that purpose, powered by a portable nuclear reactor.