Coal mines are exceptionally dangerous places. They require constant ventilation against methane accumulation and pumping out water that seeps in all the time. The soil is friable and there is a careful trade-off between personel safety and resources invested into consolidation and securing the galleries against collapse.
A mine is not some kind of free underground cave, it's an industrial installation requiring constant engineering. They have 24/7 command centers that monitor and correct safety parameters before anyone is allowed to go in. When mines close, they are flooded, refilled or sealed, to prevent underground explosions and collapse that could kill people above. The closure of mines is itself a dangerous and expensive operation.
Bottom line, if it ain't worth going down there and picking unlimited quantities of coal worth $70/ton, it most certainly isn't worth it to invest and labour for months in a high risk environment, only to get limited quantities of agricultural produce worth $200-$300/ton.
A mine is not some kind of free underground cave, it's an industrial installation requiring constant engineering. They have 24/7 command centers that monitor and correct safety parameters before anyone is allowed to go in. When mines close, they are flooded, refilled or sealed, to prevent underground explosions and collapse that could kill people above. The closure of mines is itself a dangerous and expensive operation.
Bottom line, if it ain't worth going down there and picking unlimited quantities of coal worth $70/ton, it most certainly isn't worth it to invest and labour for months in a high risk environment, only to get limited quantities of agricultural produce worth $200-$300/ton.