Whether it's gun running, drug smuggling or human trafficking, crime has one common currency: Cold, hard cash.

Immigration and Customs Enforcement agents recently completed the largest cash-smuggling sting in history, a five-day operation involving police in 83 countries that uncovered more than $27 million in undeclared at airports and border crossings across the globe.

Operation ATLAS — short for Assess, Target, Link, Analyze and Share — was hailed by Homeland Security Secretary Janet Napolitano as an "unprecedented" model of international cooperation — and one that snared $3.5 million in cash at airports and points of entry.

"People have to get the profits back (from illegal dealings abroad) and they can't use a bank, and that's what we're trying to go after," said John Morton, assistant secretary of homeland security for U.S. Immigration and Customs Enforcement.

"As you know, when you hit someone in their pocketbook, it hurts a lot."

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Under most international regulations, travelers may cross borders carrying over $10,000 as long as the money is declared. But cash couriers for criminal enterprises don't want their names ending up on government databases, which would allow police to track their patterns of travel. So many simply hide the money and take the risk.

From wads of bills in a suitcase to cash stuffed in truckloads of sugar — customs agents have seen it all. Criminals have even hidden money in women's tights, or rolled up in packs of lifesavers, or concealed in cartons of cigarettes and platform heels. One desperate man swallowed $2,000 in condom, but the bills eventually had to come out.

"Traditional methods of smuggling in their suitcases as well as on their body is still prominent," says Thomas Winkowski of U.S. Customs and Border Protection. "Operation ATLAS is focused on ten international airports throughout the U.S."

Detroit led the U.S. last year with 128 seizures, followed by New York, Houston, Chicago and Boston. Seizures in Operation ATLAS include silver coins concealed in a rice cooker and cash woven into winter jackets. The majority of cash came in U.S. dollars, but also included pounds and Euros.

And while Southern California may be a gateway for drugs coming from Latin America, sources say most couriers don't head straight from LAX to Mexico City or South America. Instead, they make detours to Hong Kong or Canada to throw police off their trail and make multiple runs with relatively small amounts to protect cartel profits.

All these tricks of the trade, say Homeland Security agents, are reasons the agency has made cutting off their cashflow an international effort.

William La Jeunesse joined FOX News Channel (FNC) in March 1998 and currently serves as a Los Angeles-based correspondent.