The secret CIA memo was couched in bland bureaucrat-ese, but the subject could not have failed to cause alarm: Surveillance aircraft had spotted underground bunkers being built in a country with a known interest in nuclear weapons. “Overhead photography has confirmed reports of extensive underground construction,” the memo warned.

U.S. concern about the possibility of underground bunkers in Cuba and other countries goes back decades. Above, a file photo of Cuban leader Fidel Castro and Soviet Premier Nikita Khrushchev in 1960. (Marty Lederhandler/ASSOCIATED PRESS)

But the country in question was not Iran, but Cuba, and the memo was written in 1966, decades before the current fascination over Iran’s fortified nuclear plants. The document is among 40 government memos and studies related to bunker-busting that were published online last week, highlighting 60 years of intense U.S. interest in the science of detecting and destroying deeply buried targets.

While military planners today are focused on Iran, the trove of documents show U.S. officials grappling with the bunker problem as far back as World War II. Over the years, U.S. agencies tracked more than 10,000 potential targets, ranging from Soviet missile silos to an elaborate mountain bunker built for former North Korean leader Kim Il-Sung, according to the records released by the National Security Archive, a nonprofit institution that publishes formerly classified materials.

“It is clear that no other nation, including Israel, can match the United States’ capabilities for the collection and analysis of data” on such bunkers, or the ability to make weapons to destroy them, the Archives said in an analysis of the documents.

Many of the documents reflect deep worries over whether an adversary might some day surprise U.S. officials with new weapons or capabilities developed in buried facilities invisible to U.S. spy satellites. Intelligence agencies over the years explored and tested a number of novel technologies for spotting underground bunkers, ranging from special cameras that could detect subtle disturbances in the natural landscape, to sensors that could pick up vibrations from mechanical equipment buried underground.

One system, described in a 1999 study, used technology known as “laser vibrometry.” It employed special lasers carried on aircraft that could detect minute mechanical vibrations, such as those that might be given off by an air-conditioning vent or an exhaust shaft leading to an underground weapons factory. Another military project used studied seismic waves to detect chambers concealed beneath layers of rock.

The effectiveness of such systems remain classified, as do the performance specs of the Air Force’s newest and largest bunker-busting bomb, the 30,000-pound Massive Ordnance Penetrator, known as MOP.

Pentagon officials recently announced yet another research project: one to make the MOP even stronger. While U.S. officials are increasingly confident of their ability to find hidden bunkers, recent tests against simulated bunkers reportedly failed to persuade military commanders that the MOP could bore through 300 feet of rock, the estimated depth of Iran’s newest bunker, built into a mountain near the ancient city of Qom.

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