Hedge-fund manager John Paulson made $1 billion using a complex financial instrument to pump up a bet that the subprime-mortgage market would crater. The parent company of retail giant Sears made $74 million using a similar device to boost its wager that a basket of stocks would rise in value.

Both were playing with leverage -- the magical power that allows investors to make big investments without putting big money on the table. These days, they have lots of company. Thanks to advances in financial engineering, investors have never had so many different ways to make commitments that exceed their bankrolls. And never before has leverage wormed its way into so many nooks of the financial world.

We're living on planet leverage, and regulators and market gurus are growing nervous.

WOULD-BE share punters, keen for a piece of China's booming stock market, are queuing to open accounts at a Beijing branch of China Merchants Securities. A busy manager, handing out application forms, says he is taking on 100 new clients a day, perhaps five times as many as a year ago. Bunches of small investors, ranging from students to pensioners, crowd around computer terminals to carry out their trades, keeping an eye on the prices as they flicker across big electronic screens. China's biggest-ever stock market boom may be turning into a bubble—and the country's leaders are getting worried.

If the bubble were to pop, it could have a bigger impact on social stability than any previous downturn in the stock market's 16-year history. There are now more than 91 million accounts held by individuals at brokers or in mutual funds.