The dark web is known for illicit activity. And in the past couple months, it has been facing its share of challenges, with two of its biggest marketplaces–AlphaBay and Hansa–being shut down by international law enforcement back in July. Now, today, four of its largest drug marketplaces have gone down.

The dark web drug trade–led by Dream, Tochka, Trade Route, and Wall Street–provides a mail-order service for whatever drug a person might want. As Joseph Cox reports in The Daily Beast, one site going offline is not unusual. But having all four key markets suddenly go dark at the same time is a signal that something’s up.

DDoS attacks appear to be the cause of the dark web going going darker.

Read the full story in The Daily Beast.