The FBI documents that accompanied last week's arrest of 10 alleged Russian spies are alternately creepy—who knew the Tribeca Barnes & Noble was a hotbed of espionage?—and comical—turns out even foreign spies wanted to cash in on suburban New Jersey's real estate boom. With a nod to Boris and Natasha, the accused are also said to have used short-wave radio, a 1920s-era technology that, because of its particular place in the spectrum, can bounce off the atmosphere and travel across continents. The FBI's criminal complaint paints a picture of stateside spies hunkered down in front of their radios, year after year, in homes in Montclair, N.J.; Yonkers, N.Y.; Boston; and Seattle, furiously filling spiral notebooks with "apparently random columns of numbers" broadcast from the motherland.

Just as in the case of Cuban spies Walter and Gwendolyn Myers, arrested last summer in Washington, D.C., the clandestine Russian agents were tuning in to foreign short-wave stations transmitting strings of numbers—some in Morse code, others spoken by a recorded voice—that they then decoded into words. The so-called "numbers stations" carried regular broadcasts that could be heard by virtually anyone across the United States spinning their own short-wave dial past the BBC World Service or Radio France International, two of many neighbors in the shortwave spectrum.

It may seem like the digital era of spy technology has passed the Russians by. In the Washington Post, columnist Jeff Stein tittered that "the FBI must have been clapping its collective hands when it discovered the primitive radio techniques the Russians were using." But they aren't the only ones using short-wave radio for espionage. Great Britain has publicly admitted that its foreign intelligence agency, MI6, still uses "numbers" stations. And scientists have tracked numbers broadcasts to transmitters at government sites in Israel and (until they went silent in the late '90s) the United States. Here are two examples of what they sound like, from the Conet Project:

Recorded in July 1994 and believed to be an MI6 station.

Recorded in December 1991 and believed to be a KGB station.

The reason this dusty method is still ideal for espionage is that, even if you locate a spy station's transmitter, you have no idea who's tuning in across the hemisphere. Unlike telephone or Internet connections, receiving a radio signal leaves no fingerprint, no traceable phone connection, no IP address, and no other hint as to where the recipient might be. The omnipresence of these "numbers stations" has engendered a community of eavesdroppers who pinpoint the stations and even take their own stab at unraveling the messages—guys like 43-year-old Baltimore-area computer engineer Chris Smolinski. When he is not running his appropriately named Black Cat Systems software firm, he's managing the Spooks list, an online gathering of several hundred amateur spy-radio buffs from around the world, all carefully scanning the short-wave bands and logging the daily bursts of numbers that fill the ether.

Smolinski has a fairly elaborate roomful of gear, complete with a humongous outdoor antenna that one can imagine has inspired suspicious neighbors to call the FBI about him. But you hardly need such an expensive set-up to channel your inner James Bond. "A little $49 handheld short-wave unit can pick up any of the Cuban stations," he says. Just tune in to 7887 kHz at the scheduled time and, clear as a bell, after an introductory "Atención!"you'll hear a female voice reading off sets of numbers. [Example below.]

Recorded in February 1995 on one of several stations confirmed by the FBI to be used by Cuban spies operating in the U.S.

With the precise numbers-to-letters code known only to a spy and his boss—and with the code intended to shift with each new broadcast—the encrypted messages are theoretically almost unbreakable. Unless, as in the famed 1946 "Venona" case proving Stalin's infiltration of the U.S. atomic bomb project, an agent clumsily reuses a "one-time" code. Or, if the FBI surreptitiously enters the spies' homes and copies their decryption keys, as occurred during this latest case of Russian espionage and prior to the 1998 Miami arrest of Cuba's "Wasp Network."

Smolinski's circle doesn't have the resources of the FBI. But that hasn't stopped them from sleuthing away on their own. And thanks to the declassification of many of the Wasp Network's decrypted messages, Spooks devotees were able to verify one of their amateur decryption efforts—the code announcing an imminent rebroadcast of a Havana-to-Miami transmission if it initially went out garbled. As in the Venona episode, the sloppy repetition of a one-time code unlocked the key. This sort of slapdash operation is endemic to Cuban spy radio.

After 50 years of communism, Fidel's black-ops are, like much of Cuban society, barely holding together. "They make so many errors," Smolinski says. "Forget about supercomputers—with the Cubans I have visions of punch cards." The result? Broadcasts that sometimes play at the wrong speed or backward, or cut out midway. Radio Havana Cuba—one of the island's main outlets, with which the spy station apparently shares facilities—is sometimes patched in accidentally. Listeners have lately heard a Venezuelan state-run station interrupting Cuba's spy broadcasts. Is a thick-fingered operative trying to multitask while monitoring the latest news from Caracas? Or maybe just adding a dash of mysterious color to the odd world of spy radio?

Akin to an identifying password, each numbers station has its own eerily unique signature, ostensibly to help an agent tune it in. A vintage 1971 broadcast, thought to originate from East Germany's Stasi, opens with a rousing beer-hall polka and the Communist anthem "The Internationale" before continuing with the numerology. Magnetic Fields, a station whose origins are still puzzled over, begins with Jean-Michel Jarre's synthesized New Age tune "Les Chants Magnétique" before airing strings of Arabic numerals and the English phrase "again, again." The broadcast recorded from Moscow during the aborted 1993 Communist Party coup against Boris Yeltsin sounded a more ominous note: the number 5 repeated over and over for hours. Listen to the abridged clips below, respectively.

Recorded in 1971 and believed to be an East German Stasi station.

Recorded in the mid-'90s, this Middle Eastern station's precise country of origin is unknown.

Recorded off a Moscow-based station during the 1993 hard-line Communist Party coup attempt against President Boris Yeltsin.

"I've always wondered why our side stopped doing it," says John Fulford, a 62-year-old Spooks devotee and ham radio operator in West Palm Beach, Fla. Now semi-retired, Fulford spent the early '80s as a law enforcement official on South Florida's narcotics beat, tuning in to drug smugglers on the short-wave band. "They'd use it to communicate between trawlers off-shore and the coast. They'd be very open about it: 'We have a box of bananas coming in very ripe!' " When off-duty, Fulford kept his short-wave radio on, traveling up and down the Florida coast with his late friend William Godby, a retired Naval Intelligence officer and budding Spooks-ologist. The pair used signal direction finding equipment to track homegrown numbers station transmitters to locations ranging from the Palm Beach International airport to the heart of Miami—and all of the stations were aiming their signals at the Caribbean.

These days, Fulford says, the radio mysteries are coming from Asia. Spooks members have recently logged new Korean numbers transmissions between Seoul and Pyongyang, as well as a Vietnamese broadcast aimed at California.

And the Russians? "They're still here," Fulford chuckles. In fact, despite last week's arrests, both Russian and Cuban numbers transmissions continue to be beamed daily to … someone. So are there more sleeper agents still sitting quietly in front of their radios across America? Smolinski says to bet on it: "The assumption is that if they're bothering to be on the air, there must be someone out there listening in on the other end."

Like Slate on Facebook. Follow us on Twitter.