With the recent hack of the U.S. Office of Personnel Management affecting approximately 22 million people, there's plenty of reason to worry about cybersecurity.
Adversaries are engaging the U.S. in cyber-war on hidden battlefields where they target military, government, banks and private businesses. The greatest threats come from China, Russia, Iran and North Korea, according to The New York Times.
Sometimes hackers are backed by nation states. Sometimes hackers are individual actors — lone wolves who get their jollies from creating digital mayhem. This video picked out five hackers as "the most dangerous of all time."