Illegal immigrants for the most part, they aren't working under the table. They use fake social security numbers, and pay taxes (FICA) on every dollar they make. At tax time, they do not file, and end up forfeiting billions of dollars in unclaimed returns. As a side note, they also pay rent where they live, and so contribute to the local and state tax coffers that way. They pay sales tax when they buy things, which is also a major contribution. Undocumented workers aren't masked criminals. They're just people who want to work.
“We die to each other daily. What we know of other people is only our memory of the moments during which we knew them. And they have changed since then. To pretend that they and we are the same is a useful and convenient social convention which must sometimes be broken. We must also remember that at every meeting we are meeting a stranger.”—T. S. Eliot (via au-petit-matin)