BBC News Online looks at what bonds are - and why, real or fake, they might be attractive to crooks with billions to hide.

So what is a bond anyway?

At its most basic, a bond is a loan.

Governments - from the biggest nation right down to towns and villages in some countries - and companies need money to fund investment.

So they sell bonds: certificates which promise to pay the buyer a specific rate of interest, plus the return of the amount they paid for it at a fixed point in the future.

So a government looking for a billion dollars might sell, say, a million bonds each worth $1,000, offering interest of 4% a year and a lifetime of 10 years.

The interest rate will depend on who the issuer is. US Treasuries, the bonds issued to cover the US government's budget deficit, carry a very low rate, since the risk of default is almost zero. Some countries have to promise more interest because they are seen as risky.

Corporate bonds also carry different rates, depending on the strength of the company and on the amount of money they are raising.

Bonds can come in any currency that is openly exchangeable with other world currencies, although many are denoted in US dollars.

Once they have been issued, bonds can be openly traded just like a share.

Their price depends on the level of interest they carry, and the perception of the market as to how the risk has changed since they were issued. Lower risk means higher price, and vice versa.

OK, I get that. But how come there can be $6bn worth sitting in a bunch of houses raided by the police?

Just like any other security - shares in a company, for instance - the final proof that you own a bond is a piece of paper.

If you have the certificate, the bond is yours. You can sell it, use it as collateral for a loan, or even stick it under the mattress till it matures and you collect from whoever issued it in the first place.

Bonds can be traded just like shares

Most bond certificates sit in bank vaults. Many of them represent governments' foreign currency reserves, and therefore are held in central banks, but others move between traders' or financial institutions' vaults periodically as the bond they represent changes hands.

And there are a lot of them about. Almost $10 trillion in all currencies outstanding at the last count. Hundreds of billions of dollars worth get traded every day.

That, of course, makes them very attractive to people needing to conceal where their money comes from.

Even $6bn is a drop in the ocean, and can easily be amassed - over a discreet period of time - by brokers working out of offshore centres in the Caribbean or the Pacific.

Then, particularly if they're in a variety of currencies and from various sources, it's not too difficult to feed them back into the financial system and to recoup the money.

Perfect for the well-heeled but careful crook with a lot of loot to launder - and to feed back into funding criminal activities.

Aren't the ones seized meant to be fakes?

We don't know yet. The police are checking, but for the moment they're not telling.

Bond certificates are a lot easier to carry

If they are, then obviously that's a very cost-effective way of making money - although they'd have to be pretty good fakes, since people tend to be careful when looking at pieces of paper worth $10,000 or more.

Moreover, if they are forgeries, they wouldn't be much use as a direct payoff for drugs, since the recipient wouldn't be best pleased.

Instead, the counterfeits could be the payoff for drugs of a much lower face value than $6bn - easier than raising the cash itself, and safer for both sides of the deal. And the recipient can simply resell them as if they were real.

Because of the size of the market, even if they do prove to be fakes, the sheer scale of bond trading worldwide means the market won't suffer.