devoid or partially devoid of light or brightness; shadowed or black or somber-colored; "sitting in a dark corner"; "a dark day"; "dark shadows"; "the theater is dark on Mondays"; "dark as the inside of a black cat"
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hidden
difficult to find; "hidden valleys"; "a hidden cave"; "an obscure retreat"
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invisible
impossible or nearly impossible to see; imperceptible by the eye; "the invisible man"; "invisible rays"; "an invisible hinge"; "invisible mending"
