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Station Hypo

Diorama by Norman Bel Geddes, depicting the lead elements of Marine Fighting Squadron 221 (VMF-221) intercepting the Japanese air strike formation headed toward Midway on the morning of June 4, 1942. Planes in the foreground are F4F-3 “Wildcat” fighters. Official U.S. Navy photograph National Archives

Just months before the infamous attack on Pearl Harbor, in the summer of 1941 a small collection of intelligence analysts, linguists and code-breakers would set up the newly renamed Combat Intelligence Unit in the nondescript basement of building 1, the administrative headquarters of the 14th Naval District.

The unit was also known by the code name Station Hypo. Hypo was charged with the collection and analysis of Japanese radio signals by which the United States hoped to keep a wary eye on the expansionist designs of the Japanese Empire.

The basement, affectionately known as the “dungeon,” was described as a large windowless space crammed with tables full of boxes containing cards and printouts from tabulating machines that would continually spit out messages intercepted from the various receiving stations around the Pacific. There was one entrance in and out guarded at all times by an armed Marine.

Hypo was led by a brilliant officer named Cmdr. Joseph Rochefort who worked tirelessly with his small staff to keep abreast of Japanese intentions. However, in 1941 the American code-breakers were still unable to read the Japanese operational code known as JN-25 and instead were forced to rely on diplomatic traffic, the inconclusive observation of ship movements and routine radio correspondence. As a result, the United States was caught by surprise when Japan launched their raid on the morning of Dec. 7, 1941. But Hypo’s finest hour was yet to come.

By March of 1942 the JN-25 code had been broken and Rochefort and his staff were determined to keep close tabs on Japan’s military forces and stay one step ahead of the enemy’s intentions. From the “dungeon” it was determined that the Japanese would launch an invasion of an undisclosed location known only as “AF.”

Rochefort reasoned that “AF” was Midway Island and that the Japanese would attempt an invasion around June 4. Many in Washington remained convinced that Japan’s next move would be toward the Aleutians, leaving Adm. Chester W. Nimitz with the difficult decision of determining how to best use his limited forces.

Once again the answer came from the basement of building 1 when a brilliant scheme was devised to expose the enemy. A message was sent out across the undersea cable instructing U.S. forces on Midway to send a false radio message in the clear stating that Midway’s fresh water system had malfunctioned.

The Japanese took the bait and soon Hypo intercepted a Japanese message stating that “AF” was having trouble with its fresh water system. Soon Nimitz gave the order to send his available carriers north of Midway to intercept the Japanese strike force and a decisive battle was won, as much by the intelligent exploitation of perceived Japanese intentions as it was by the pilots and crew who sunk four of Japan’s fleet carriers.

In April 1943 the Combat Intelligence Unit would move out of its cramped facilities at building 1 and move into a new home at Makalapa near the Pacific Fleet headquarters. The “dungeon” would lie vacant, serve as storage spaces or administrative offices for the next several decades until its legacy was rediscovered in the last few years. There are now two entrances into the space and a plaque hangs outside each stairway to mark the important events that happened in the darkest days of America’s struggle against Imperial Japan.

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