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PROPERTY, PLANT AND EQUIPMENT | 12 Months Ended
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Dec. 31, 2023
PROPERTY, PLANT AND EQUIPMENT |
PROPERTY, PLANT AND EQUIPMENT | 5.PROPERTY, PLANT AND EQUIPMENT ​ Property, plant and equipment consist of the following: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, |
​ | ​ | 2023 |  | 2022 | ​
​ | ​ | (in thousands) | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Mining equipment and processing facilities | ​ | $ | 1,989,541 | ​ | $ | 1,927,603 | ​
Land and coal mineral rights | ​ |  | 504,736 | ​ |  | 499,950 | ​
Oil & gas mineral interests | ​ | ​ | 853,350 | ​ | ​ | 814,667 | ​
Buildings, office equipment, improvements and other miscellaneous equipment | ​ |  | 310,876 | ​ |  | 300,436 | ​
Construction, mine development and other projects in progress | ​ |  | 184,895 | ​ |  | 99,042 | ​
Mine development costs | ​ |  | 329,146 | ​ |  | 289,724 | ​
Property, plant and equipment, at cost | ​ |  | 4,172,544 | ​ |  | 3,931,422 | ​
Less accumulated depreciation, depletion and amortization | ​ |  | (2,149,881) | ​ |  | (2,050,754) | ​
Total property, plant and equipment, net | ​ | $ | 2,022,663 | ​ | $ | 1,880,668 | ​
​ All of our property, plant and equipment have depreciable lives of 1 to 20 years. Depreciation, depletion and amortization expense related to property, plant and equipment was $276.4 million, $273.8 million and $260.3 million for the years ended December 31, 2023, 2022 and 2021, respectively. ​ At December 31, 2023 and 2022, land and coal mineral rights above include $13.4 million and $29.9 million, respectively, of carrying value associated with coal mineral reserves and resources attributable to properties where we or a third party to which we lease coal mineral reserves and resources are not currently engaged in mining operations or leasing to third parties, and therefore, the coal mineral reserves are not currently being depleted. We believe that the carrying value of these coal mineral reserves will be recovered. ​ At December 31, 2023 and 2022, our oil & gas mineral interests noted in the table above include the carrying value of our unproved oil & gas mineral interests totaling $411.6 million and $422.7 million, respectively. We generally do not record depletion expense for our unproved oil & gas mineral interests; however, we do review for impairment as needed throughout the year. ​ During 2023, we incurred $44.4 million in mine development costs, primarily related to Tunnel Ridge and the Henderson County mine at River View Coal, LLC ("River View"). During 2022, we incurred $11.3 million in mine development costs, primarily related to Hamilton and River View mine. All past capitalized mine development costs are associated with other mines that shifted to the production phase in past years and we are amortizing these costs accordingly. We believe that the carrying value of the past development costs will be recovered.

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