CASEY SERIN knows all about the excesses of America's housing bubble. In 2006 the 24-year old web designer from Sacramento bought seven houses in five months. He lied about his income on “no document” loans and was not asked for anything so old-fashioned as a deposit. Today Mr Serin has debts of $2.2m. Three of his houses have been repossessed; others could share that fate. His website, Iamfacingforeclosure.com, has become a magnet for those whose mortgages are in trouble.Mr Serin and people like him are Wall Street's biggest uncertainty just now. How many Americans are saddled with mortgages they cannot afford on houses that are losing value? The answer matters to anyone who bought high-yielding mortgage-backed securities when a booming property market made mortgages look safe. It also matters to investment banks, which packaged the securities and often own subsidiaries that originate mortgages. It may determine whether America's economy falls into recession. It could even affect the outcome of next year's elections.