What happens on the dark web is so ugly that cybersecurity firms that comb its data routinely share the information with the FBI and other law enforcement agencies. "All us work in partnership with law enforcement, when possible and necessary, to combat this," said Danny Rogers, chief executive of Terbium Labs, a Baltimore company that specializes in automated combing of the dark web. A handful of U.S. cybersecurity companies focus on the dark web, primarily working for retailers, banks and other firms concerned that cybercriminal gangs are trafficking there in consumer data that they've obtained through breaches. Those visiting the dark web must employ special web software, like The Onion Router (Tor) or I2P, both of which encrypt and give anonymity to the user and hide the location of everything visited. Two weeks ago, the FBI and law enforcement agents from six other countries took down the dark web's biggest criminal marketplace, AlphaBay, which it called "the largest criminal marketplace on the internet." The dark web, sometimes also called the dark net, is a hive for cybercriminals involved in buying and selling stolen personal information.