International authorities shut down an online marketplace and arrested nearly 300 people who allegedly used it and other parts of the "dark web" to buy and sell fentanyl and other dangerous opioids in a sweep officials said underscores how hard it is to stem the tide of drug trafficking in the internet’s hidden corners. The operation went on for more than 18 months and spanned three continents, U.S. officials said Tuesday, the Wall Street Journal reports. Law-enforcement agencies seized more than $53 million in cash and virtual currencies, along with guns and nearly 2,000 pounds of drugs. A focus of the effort was the potent synthetic opioid fentanyl. The drug’s inexpensive, easy-to-replicate formula has boosted its appeal among criminal networks, fueling an overdose crisis that claimed a record 106,700 lives in the U.S. in 2021. The bust was the latest of its kind by U.S. and allied law-enforcement agencies, which have sought, with limited success, to shut down darknet forums to curb the rash of drug trafficking, ransomware attacks and other criminal activity. Security experts say criminals quickly find another marketplace shortly after one goes under. The 2013 seizure of Silk Road was considered a watershed operation at the time, but criminals soon found a similar option in the AlphaBay marketplace until it, too, was knocked offline in a law-enforcement operation in 2017. Attorney General Merrick Garland acknowledged that criminals are often able to reconstitute quickly but said that the expanding law-enforcement pressure was making it harder for replacement marketplaces to start. “There is a bit of a whack-a-mole problem here, and we are whacking as hard as we can,” Garland said. “We do believe it is having an impact.” The Justice Department’s operation was coordinated with other agencies including Europol, which said European law-enforcement authorities seized “Monopoly Market,” a marketplace operating on the dark web, a network of websites that uses anonymity software to hide users’ locations. Customers on darknet forums use cryptocurrency to pay for illegal goods or services such as malware for cyberattacks, making it difficult for investigators to track them down. The investigation began in October 2021 and resulted in 153 U.S. arrests., including a California man who allegedly used at least nine different marketplaces and employed a network of workers to ship millions of pills laced with fentanyl and methamphetamine to darknet buyers.