The law was written so that any surplus from the social security tax was put into the general fund, and the social security trust fund was given a special treasury note that could not be redeem or sold to anyone else then the federal government. This was done because of two reasons; 1. people did not trust the stock market at that time (great depression)2.economist thought that putting that much money into the stock market would cause massive problems.
It's in a "lock box" that idiot Al Gore informed us in one of his debates with GW Bush.
Al never did tell us where this infamous "lock box" was nor did he admit that the federal government was the only one with a key to this so-called "lock box" and that they managed to pilfer all of the money away by mixing it in with operating funds received from the taxpayers.