The underground Palestinian economy

Half the US economy is "underground."

I would like to believe this figure--do you have any references?

The Wikipedia page on Underground Economy refered to an article from 2005 that claimed 9% in its open paragraph:

America has two economies, and one is flourishing at the expense of the other. First, there's the legitimate economy, in which craftsmen are licensed and employers and employees pay taxes. Then there's the fast-growing underground economy, where millions of nannies, construction workers and others are paid off-the-books, their incomes largely untaxed. The best guess as to the size of the output of this shadow economy is about $970 billion, or nearly 9% that of the real economy. It should soon pass $1 trillion.

By the way, the article continued with a naive statist bias:

In the process, the underground economy is undermining the effectiveness of the Internal Revenue Service, which is highly dependent on employees' withholding taxes. If the IRS could collect all the taxes it says that it is owed from the underground economy in a given year, then the current budget deficit would disappear overnight. And if the IRS could collect these taxes every year, then the nation would have surpluses as far as the eye can see.

I thought the idea was a little ridiculous that if the government increased its tax collection that they would stop borrowing or creating money ex nihilo and be content to keep the budget at its current size. I work on the assumption that those in government extract as much resources out of the economy as they possibly can.