What is the ruling on a mine developed by an unbeliever in enemy territory if the land is later conquered by Muslims?

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If an unbeliever excavates a mine in the territory of war (dar al-harb) and reaches the water source, and subsequently Muslims conquer the territory by force (anwah), the mine does not become spoils of war (ghanima). The unbeliever's efforts are rendered null. This is because the excavator did not gain ownership through the act, and if ownership were established, the entire land would become an endowment (waqf) for the Muslims, designated for their communal benefit, just as if the mine had appeared through divine act.