What happens if you make the underground economy legal?

The underground economy refers to economic transactions that are deemed illegal, either because the goods or services traded are unlawful in nature, or because transactions fail to comply with governmental reporting requirements.

Illegal economy refers to a type of economy where income is produced by those economic activities pursued in violation of legal statutes defining the scope of legitimate forms of commerce ... In illegal economy, participants engage in the production and distribution of prohibited goods and services.