Questions 56 to 63 are based on the following passage. 　　In many businesses, computers have largely replaced paperwork, because they are fast, flexible, and do not make mistakes. As one banker said, “Unlike humans, computers never have a bad day.” And they are honest. Many banks advertise that their transactions are “untouched by human hands” and therefore safe from human temptation （诱惑）. Obviously, computers have no reason to steal money. But they also have no conscience （良知）, and the growing number of computer crimes shows they can be used to steal. 　　Computer criminals don’t use guns. And even if they are caught, it is hard to punish them because there are no witness and often no evidence. A computer cannot remember who used it: it simply does what it is told. The head teller at a New York bank used a computer to steal more than one and a half billion dollars in just four years. No one noticed this theft because he moved the money from one account to another. Each time a customer he had robbed questioned the balance in his account, the teller claimed a computer error, then replaced the missing money from someone else’s account. This man was caught only because he was a gambler. When the police broke up an illegal gambling operation, his name was in the records. 　　Some employees use the computer’s power to get revenge （报复） on their employers they consider unfair. Recently, a large insurance company fired its computer-tape librarian for reasons that involved her personal rather than her professional life. She was given thirty days notice. In those thirty days, she erased all the firm’s computerized records. 　　Most computer criminals have been minor employees. Now police wonder if this is “the tip of the iceberg”. As one official says, “I have the feeling that there is more crime out there than we are catching. What we are seeing now is all so poorly done. I wonder what the real experts are doing — the ones who know how a computer works.” Questions: The phrase “the tip of the iceberg” roughly means ________.