And we all thought crypto was untraceable. Well, that's not the case for one South Korean millennial who was busted by U.S. and Korean authorities for running the world's "largest dark web child porn marketplace."

The now seized website, called "Welcome to Video," operated on the mysterious Darknet that only accepted cryptocurrency and contained 200,000 videos of sexually explicit videos involving children, according to an 18-page criminal indictment viewed by NBC News.

According to prosecutors, Jong Woo Son, 23, a South Korean citizen, who ran the site from the summer of 2015 until it was seized by U.S. authorities in March 2018, is currently serving an 18-month prison sentence in South Korea after being convicted for child pornography.

At a press conference on Wednesday, U.S. Attorney Jessie Liu and other officials said 337 site users had been arrested around the world, including many from the U.S., Brazil, Germany, Saudi Arabia, and the United Arab Emirates.

"You may try to hide behind technology," Liu said, "but we will find you and arrest you and prosecute you.

The site, which operated on the Darknet, encouraged many of its users to upload videos and tens of thousands of illicit pictures.

Cryptocurrency, such as Bitcoin, was widely used by users to buy and sell child porn on the website.

National Center for Missing and Exploited Children, Deputy Assistant Attorney General Richard Downing, said at least half of the content on the website was never before seen. "There are many children depicted in these videos that have not been identified."

"The scale of this crime is eye-popping and sickening," said John D. Fort, chief of IRS criminal investigations.

U.S. investigators said back in 2017, during an examination of the site, they were able to reveal the exact I.P. address and trace it back to Son's home, where he had servers in his bedroom.

A criminal probe allowed authorities to make controlled buys of child porn videos from the site. Investigators also unmasked users hiding behind cryptocurrency transactions, Fort said.

"Our agency's ability to analyze the blockchain and de-anonymize Bitcoin transactions allowed for the identification of hundreds of predators around the world," he said.

The Korean National Police Agency said, "most of the users were in their 20s, unmarried and white-collar office workers and first-time offenders, although some were ex-convicts of sexual crimes, including juvenile sex offenders. One possessed as many as 48,634 child porn [files]."