This is probably the longest “six piece tablebase endgame puzzle” ever: a mate in 262 moves! Lewis Stiller found out in 1991 that this endgame, a King plus Rook and Knight versus King plus two Knights, is a win for the strongest side in 78% of all cases. The 262 move solution is credited to Ken Thompson who constructed the 93 gigabyte database and Peter Karrer, who found this longest mate in it. Tim Krabbé’s comments magnificently describe it: “The moves are awesomely beautiful. Playing over these moves is an eerie experience, they are not human. A Grandmaster does not understand them any better than someone who has learned Chess yesterday. The Knights jump, the Kings orbit, the sun goes down, and every move is the truth.”