Richard McKenna. The Secret Place.
(Nebula Award, Best Short Story. 1966.) This story was found amongst the author's papers after his death. The question "What did you do in the war daddy?" causes Duard to look back on his wartime experience, which is somewhat more obscure than most. Barker, Oregon, was the site of a geological survey, which sought to identify a possible source for a crystal of uranium oxide found by the body of a young man back in 1931.
Duard is left behind to do the final surveying, and local myth surrounding a lost mine captures his attention, as does the sister of Owen, the young man whose body set off the search. The girl, Helen, has only recently come out of a mute period, and Duard finds her difficult to communicate with.
But, out in the desert, he unearths the fantasy worlds which Helen and Owen played in as young people. Bizarrely, the consistent topography of their fantasy world is revealed to match the lower geological structures of the area.