We are confronted with artifacts we do not understand, from a race or culture that's not around to tell us what it all means. Sometimes it's a human culture; sometimes it's alien. Enigma tales often involve the promise of great wealth to lure the main characters into action, and the puzzles can be the author's way to illuminate the characters confronting them. Some have argued that, since SF is the literature of "What If?" Enigma Tales represent one of the purest expressions of the genre.
Rogue Moon, Algis Budrys
Gateway, Fredrick Pohl
"Ticket to Anywhere," Damon Knight
"By The Waters of Babylon," Stephen Vincent Benét
“Mimsy Were the Borogoves,” Henry Kuttner and C.L. Moore (writing as Lewis Padgett)
Forbidden Planet, film, Irving Block, Allen Adler, Cyril Hume, Fred M. Wilcox
Solaris, Stanislaw Lem