By A.S. Berman
Decades after the Cuban missile crisis sent a generation of schoolchildren scurrying to ''duck and cover'' beneath their desks, strange apparitions of that era linger on.
Long into the night, these phantoms babble incessantly over shortwave (also known as world band) radios, emitting strange sounds and rattling off what seem like random numbers.
Thought to be coded instructions aimed at foreign and U.S. spies in the field, these enigmatic broadcasts are rapidly being stripped of their mystery, thanks to the World Wide Web.
SpyNumbers.com is just one site documenting these ''numbers stations,'' offering searchable profiles of each station that include sound descriptions, broadcast times and station frequencies.
The Web, says SpyNumbers' Chris Smolinkski, has allowed amateur researchers of this phenomenon not only to share station frequencies, but also to determine the best times to tune them in.
The site even features links to streaming audio clips of the broadcasts so that the curious can sample the signals without having to plug in a radio.
''Someone in Europe can hear (a station), digitize it and send you a (sound) file'' in minutes, says Smolinkski, 33, a Westminster, Md., electrical engineer. ''It's just like being there.''
Hugh Stegman, an editor at the shortwave-radio journal Monitoring Times, says numbers stations first hit the airwaves around the time of the Cuban missile crisis, the 1962 nuclear standoff between the Soviet Union and the United States.
Shortwave, he maintains, remains the best means for giving spies their marching orders worldwide, despite the Internet and other technological advances.
Modern technology ''is very good, but it's expensive,'' says Stegman, 45, of Los Angeles. ''You don't want to go leaving it around in Botswana. (Also), if you put a satellite dish up . . . people want to know what you're doing.''
Shortwave radio receivers, on the other hand, are a common sight throughout the world.
One of Stegman's favorite numbers stations, long since off the air, featured a Cuban man who would hit the airwaves around midnight.
''This guy would come on and read off numbers. He sounded very hyper, like he was stoned on speed. He'd read them so fast, I don't think anyone could understand him.''
Indeed, Cuban numbers stations are routinely ridiculed in shortwave circles for their slipshod, Laurel-and-Hardy-like performances.
''They probably run about as well as most things in Cuba,'' Smolinkski says. ''They share a transmitter site with Radio Havana, and sometimes the numbers station gets the Radio Havana audio. Maybe Castro gets all the hand-me-downs from Russia.''
Although several Web sites are dedicated to the phenomenon, U.S. government agencies only grudgingly acknowledge the broadcasts even take place.
''We know of their existence,'' says John Winston, assistant chief of the Federal Communications Commission's enforcement bureau. ''There are those both in and outside the country.''
The FCC only investigates such signals if they interfere with domestic radio broadcasts, which happens ''very rarely,'' he says.
Many believe that America's own CIA operates a numbers-station transmitter at Virginia's Warrenton Training Center, just outside Washington, D.C.
Not surprisingly, both the CIA and the top-secret communications center declined to comment.
However, John Pike, a defense analyst at Washington's Federation of American Scientists, visited the military site. He came away ''satisfied'' that the complex is the source of at least one of the mysterious broadcasts.
''I have very little reason to doubt it,'' Pike says. ''They're the only big transmitters anywhere around there.''
Though Smolinkski would welcome an explanation of the numbers stations from the U.S. government, he doubts that information will be forthcoming.
In the meantime, he's content with scrutinizing entries that continue to be made to SpyNumbers' ever-growing online database.
America's supersecret National Security Agency ''probably has a much better database,'' he acknowledges with a laugh. ''But they get paid for it.''