SEOUL—Yoo Young-bok's worn hands and gnarled fingernails are the only visual clues of his 47 years spent working in North Korean mines.

Mr. Yoo, 82 years old, is one of an estimated 24,000 South Korean prisoners-of-war that North Korea didn't repatriate after the signing of the Korean War armistice, 60 years ago in July. Most were put to work in mines in the north of the country, where many died. A few hundred are thought to still be alive in North Korea.

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