During an evening swim through big, slow swells, I floated on my back and peered across at San Francisco; the sun was sink­ing directly behind the city. All I could see was the water — dark, nearly black, the way it gets at that time of day — and the scal­loped top of the city’s sil­hou­ette, like a float­ing citadel. Atlantean, in that light. I ha

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