Two histories behind one spelling
English uses mine for what belongs to the speaker, for a working that takes ore from the ground, and for a hidden explosive. Those senses share a spelling and a sound. They do not share a past. The origin of the word mine is two stories, not one: a native Germanic possessive, and a later French borrowing for digging. The buried charge grew out of the second of those stories, not the first.
In current use the independent pronoun is the everyday possessive of I: the book is mine. An older attributive use, before a noun, survives only in set phrases and in deliberately old-fashioned style. The noun names a pit or tunnel for coal or metal, a figurative store of something useful, and, in military English, a device laid in water or soil. The verb covers extracting minerals, tunneling, laying such devices, and, figuratively, weakening something from below.
A native possessive
The pronoun is inherited, not borrowed. Old English already had mīn as both adjective and pronoun, covering the ground later split between my and mine. That form continues Proto-West Germanic *mīn and Proto-Germanic *mīnaz, the first-person singular possessive. Close cognates are still ordinary words in neighboring languages: German mein, Dutch mijn, West Frisian myn, Danish and Swedish min, with older relatives in Old Norse minn and Gothic meins.
Deeper still, the Germanic adjective is built on the same first-person stem that gives English me. Specialists reconstruct a Proto-Indo-European possessive such as *méynos, or an oblique stem *h1mey- with an adjectival suffix. The precise formula is a reconstruction, and accounts differ on the suffix. What is not in doubt is the family link: this mine is the same word as my, formed on me, and it was already in English in the Old English period, well before 1150.
Old English could also use the word as a noun for one's people or household. That sense faded. The grammatical split that matters for modern English is different. In Middle English the full form began to lose its final consonant when a noun followed. The shortened my became the usual determiner from about the thirteenth century whenever the noun was expressed. Mine remained as the independent pronoun and, for a long stretch, as the form used before a vowel or h, on the same pattern as a and an. Writers kept that prevocalic contrast into the eighteenth century. After that, my became the general attributive, and mine the form that stands alone or follows of in the double genitive: a friend of mine, no fault of mine.
The leftover -n on the independent pronoun later invited analogical copies. Dialects in southern England, and later in parts of the American South and New England, built forms such as hisn, hern, ourn, yourn, and theirn on the model of mine and thine. Standard English went the other way, adding -s to most independent possessives after the pattern of his. Mine is the one common independent possessive that never took that -s, which is why some vernaculars add it and say mines. That is analogical tidy-up, not a separate etymology.
A French word for ore and shafts
The pit-and-ore noun is a borrowing. It is recorded in English around 1300, from Old French mine, which already meant a vein or lode, a shaft or tunnel, and mineral ore. Medieval Latin mina and minera sit beside the French form; some dictionaries write a reconstructed Vulgar Latin *mina. The deeper source is treated as uncertain and probably Celtic. Welsh mwyn and Irish méin (older mein) mean ore, and a Proto-Celtic noun *mēnis or *meinis, ore or metal, is widely posited. A few reconstructions take that Celtic word from a root meaning to cut or hew, and even set it at a distance from the Germanic word for smith. Others stop at the Celtic step and call the last link a probability, not a proof. Classical Latin and Greek did not supply English with this mining term. The industry was old in Celtic-speaking regions such as Cornwall, which makes a Celtic origin for the Romance word plausible even while the last detail stays disputed.
The verb arrived in the same window, from Old French miner. Early English uses, around 1300, already include digging a tunnel under fortifications in order to bring them down. Mineral extraction in the ordinary sense is clear from the mid-fourteenth century, and the transitive sense of taking ore out of the earth from later in that century. A figurative use, to ruin something by slow or hidden means, is also fourteenth-century. The agent miner is attested even earlier, from the late thirteenth century, in the siege sense. Undermine keeps the military picture in transparent form: under plus the digging verb.
From sap to explosive
The military tunnel, recorded as a noun from about 1400, is the bridge to the modern weapon. Besiegers dug under walls, propped the cavity, then burned the supports or, later, packed the space with gunpowder. In the 1620s English to mine could mean to place such charges. The free-standing noun for an explosive device is later still. By 1866 it is used of submarine weapons, at first not sharply distinguished from torpedoes. The compound land-mine is in print by the 1870s, and the ground-laid sense is familiar by about 1890. The buried charge is named after the old siege gallery. It is not a new coinage from the possessive pronoun.
Later figurative uses of the excavation noun are straightforward extensions: a person or book can be a mine of information, a rich working rather than a hole in the ground. None of that pulls the two homonyms back together. They still collide in spelling and in pronunciation, but they do not share a root.
What the leftover -n still shows
If one dated fact is worth keeping, it is the late survival of attributive mine before vowels. For centuries English treated my and mine the way it still treats a and an. The independent pronoun never lost the -n; the determiner did. That is why the book is mine sounds ordinary, and why a phrase such as mine own now sounds like an older stage of the language rather than a second word for a coal pit. The other mine, the French one, had by then already moved from ore and shaft to sap, and from sap to the charge left in the dark.