X of the Underground was a resistance fighter opposing the Axis powers in Paris during World War II. His face never seen, his identity never revealed, the mysterious X became something of a sensation in 1942, his exploits augmented by the reports of the New York Globe's Bob Gray. Gray eventually discovered that X was a woman (and that the Underground was a multi-national force of women), but chose not to reveal the secret.
In the fall of 1942, the Gestapo believed that they had succeeded in killing X. A note found nearby the corpses of two Nazi officers told otherwise: "Notice, German murderers: X is not dead! She will live to see the last Nazi dog sent to his grave."