SCHAEFFER: The Battle Of Midway: Part Three

The Hypo “Dungeon”

U.S. cryptographers at Hawaii’s station Hypo (the British military designation for “H” for the Heeia radio post) had managed to decipher much of the Japanese Navy’s most important code, dubbed JN-25. Working in a windowless basement they called “the dungeon,” the codebreakers under the direction of the brilliant Commander Joseph Rochefort, noticed increased Japanese radio traffic referencing a point called “AF.” It was becoming clear that “AF” was the focal point of a major upcoming operation featuring a carrier group and at least one surface fleet, and it would be executed either on June 4 or 5.

But what exactly was “AF”? In one of many instances of Dame Fortune smiling on the Americans, it so happened that in March a Japanese aircraft reported its position as being near “AF”. Plotting the plane’s location, the only significant land mass in the area was Midway. Hypo had no doubts what this meant. But Washington was not so sure, fearing moves toward Australia or even Hawaii itself. To confirm that the all-important “AF” was, in fact, Midway, Rochefort’s men devised a clever ruse. They submarine-cabled instructions that Midway should radio, un-coded and in the clear, a bogus message that the base was suffering a fresh water shortage due to a malfunctioning desalinization system. The Japanese fell for it, and two days laterstation Hypo was deciphering enemy radio traffic deeming contingencies be made for extra water purification systems to accompany the landings as “AF” was suffering a fresh water shortage. “AF” had to be Midway. Now Nimitz knew where, as well as when and with what, Yamamoto would make his next move. But how to prepare for it?

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