Snark

No one has seen a snark and lived to tell the tale.

See: The Hunting of the Snark.

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In graph theory, a snark is a graph in which each vertex has three edges, but it is impossible to color the edges with only three colors so that edges of the same color don't meet at one vertex. A boojum is a planar snark. See Four-Color Theorem.

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The SM-62 Snark was an intercontinental nuclear cruise missile of the US Air Force that was operational in 1960 and 1961.