But many underground transactions are more modest.
By Peter Canby April 19, 2018 "Mlima's Tale" traces the path of a poached elephant's tusks, as they are trafficked through a series of underground transactions.
Nottage's creation, Mlima, has the enormous tusks of the real-life Satao, but, rather than focus just on Mlima's murder, Nottage made the ambitious decision to trace the stolen tusks, each step of the way along the chain of illegal underground transactions, to their ultimate destination in Beijing.
Opponents of the FairTax argue that imposing a national retail sales tax would drive transactions underground and create a vast underground economy.
In the years when construction and real-estate markets boomed, transactions conducted underground exceeded 20 percent of gross domestic product.
Silk Road was an underground online marketplace known for transactions related to illicit drug and criminal activity.
The government responds by trying to penetrate a vast underground economy, where off-the-books transactions are conducted almost entirely in cash, because it is harder for the authorities to trace and tax.
In Taiwan, commitment to bring legislation into harmony with the International Covenant on Civil and Political Rights and the International Covenant on Economic, Social and Cultural Rights resulted in anti-sex work law being abolished in 2009 on the grounds that punishing commercial sexual transactions forces sex workers underground leaving them open to abuse.
But words like informal, illicit, and underground have long been used to describe transactions that exist in a thorny gray area: not fully legal, but not explicitly harmful either.
Juan Carlos Martínez Lázaro, a professor at the IE Business School in Madrid who has studied the underground economy, said the government's decision to restrict cash transactions "isn't the magic solution but does make it harder to go ahead with a lot of undeclared transactions that now take place in cash".
