is under the table pay illegal???

There is nothing illegal about paying someone in cash per se.  However, usually the reason employers pay in cash is to cheat the taxing authorities, either so the employer will not have to pay various costs (such as unemployment insurance, etc.) and/or so that the taxing authorities will not know that the employee has earned income and the employee will not have to declare it or pay income tax on it.  If your employer did not properly report, and pay the necessary charges on, the money he paid you and others, he broke the law, and you may be able to get him in trouble by reporting him to the IRS, state taxing authority, and/or local Department of Labor.  Sometimes the IRS pays a reward to a whistleblower who reports a tax cheat. However, I don't believe that you can sue your employer personally, because you were not damaged by being paid in cash.  Moreover, you may want to think about whether your own house is in order before you take any action - did you declare and pay taxes on the cash you received from your former employer?