Iran Underground Economy's Value at Over $34b

Iran’s underground economy, according to latest data disclosed by the Ministry of Economic Affairs and Finance, was worth 5,360 trillion rials ($34.35 billion) in the year ending March 20, 2018.
In other words, the part of the economy that neither pays tax nor is monitored in any form by the government made up more than 36% of the country’s gross domestic product in the fiscal 2017-18. 
According to Investopedia, 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. It is alternately referred to as the shadow economy, the black market, or the informal economy.