Five years ago Sunday, Bernie Madoff was sentenced to 150 years in prison for running the biggest fraudulent scheme in U.S. history. Even now, only a few of his victims have since regained all of their losses.

A well-respected financier, Madoff convinced thousands of investors to hand over their savings, falsely promising consistent profits in return. He was caught in December 2008 and charged with 11 counts of fraud, money laundering, perjury, and theft.

Here's how Madoff conned his investors out of $65 billion and went undetected for decades:

Charles Ponzi

Madoff used a so-called Ponzi scheme, which lures investors in by guaranteeing unusually high returns. The name originated with Charles Ponzi, who promised 50% returns on investments in only 90 days.

Ponzi schemes are run by a central operator, who uses the money from new, incoming investors to pay off the promised returns to older ones. This makes the operation seem profitable and legitimate, even though no actual profit is being made. Meanwhile, the person behind the scheme pockets the extra money or uses it to expand the operation.

To avoid having too many investors reclaim their "profits," Ponzi schemes encourage them to stay in the game and earn even more money. The "investing strategies" used are vague and/or secretive, which schemers claim is to protect their business. Then all they need to do is tell investors how much they are making periodically, without actually providing any real returns.

Ponzi schemes aren't usually very sustainable. The setup eventually falls apart after: (1) The operator takes the remaining investment money and runs. (2) New investors become harder to find, meaning the flow of cash dies out. (3) Too many current investors begin to pull out and request their returns.

In Madoff's case, things began to deteriorate after clients requested a total of $7 billion back in returns. Unfortunately for Madoff, he only had $200 million to $300 million left to give.

Another reason Madoff managed to fly under the radar for so long (despite multiple reports to the SEC about suspicions of a Ponzi scheme), is because Madoff was a well-versed and active member of the financial industry. He started his own market maker firm in 1960 and helped launch the Nasdaq stock market. He sat on the board of National Association of Securities Dealers and advised the Securities and Exchange Commission on trading securities. It was easy to believe this 70-year-old industry veteran knew exactly what he was doing.

Madoff really only made off with $20 billion, even though on paper he cheated clients out of $65 billion, according to CNNMoney. That's hardly any consolation for his thousands of investors, the full list of whom can be found with WSJ here.

The 150-year sentence, more symbolic than literal, was followed by other convictions related to Madoff's scheme. In March this year, five of Madoff's employees were found guilty for their part in the Ponzi scheme. Most recently, Madoff's accountant and lawyer is also facing up to 30 years in prison for his role.

There are several other notable Ponzi schemes in history, including Allen Stanford's which stole $8 billion and Tom Petters' that cheated investors out of $3.7 billion. But as far as scale goes, Madoff wins by a landslide.