In 2006 and 2007, Goldman Sachs Group peddled more than $40 billion in securities backed by at least 200,000 risky home mortgages, but never told the buyers it was secretly betting that a sharp drop in U.S. housing prices would send the value of those securities plummeting.Goldman's sales and its clandestine wagers on falling home prices, completed at the brink of the housing market meltdown, enabled one of the nation's premier investment banks to pass most of its potential losses to others before a flood of mortgage loan defaults staggered the U.S.