A lucrative but highly risky undertaking

"After the number of police raids surged, we decided to turn to clients introduced by trustworthy friends and acquaintances, rather than solicit business publicly," Lin Weidong said.

Lin (not his real name) has been running an underground bank in the Luohu district of Shenzhen, a manufacturing hub in Guangdong province, for seven years.

"The pursuit of material benefits, huge market demand and loopholes in the banking supervision mechanism have been catalysts for money-laundering crimes," said the 37-year-old "black banker" from Chaozhou, Guangdong.