Ofcccc in the novel (spoiler)
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So basically the city does experiments on fetuses in order to counter the virus. And the fetuses that survived all have something enhanced such as strength or intelligence, but have birth defects. These fetuses that have been Experimented on are called “mutants”.
Dr.seok and the 3 Majors in the immortal regiment are all “mutants”. One birth defect all mutants have is they all have something they are obsessed with. For Dr.Seok it’s rocks and for Major Yang I believe it’s trains. I don’t think Major Lee’s obsession was ever mentioned. Major Kwak was known as the perfect mutant cause he didn’t have an obsession and other things I’m not gonna spoil.
But yes, the obsession with rocks is caused by the birth defect of the experimentations.
Has that been explained and i missed it??