Hackers may be classified into three, the black hat, gray hat, and the white hat hacker. However, society is more interested in the black hat, mainly because of their impact on the cyber world.

A black hat is usually considered as a hacker with the intention of doing harm. Outlined below are some of the most interesting and notable hackers in the past decades.

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Often dubbed as the first hacker, Kevin Mitnick came across with the idea of hacking when he learned the concept of social engineering and a system to bypass L.A. bus punch card system, according to Movies On Hacking. He was also charged with hacking into computer systems and illegally copying software.

When he was arrested for his cybercrimes, he was placed in solitary confinement as the judge feared that Mitnick could start a nuclear war by whistling NORAD codes via a telephone system. 

Another black hat to note is Chad Davis, responsible for vandalizing the White House website homepage. Together with his crew, Davis is considered one of the most popular cyber-criminals of the late 20th century. He was part of a team of hackers who started the globalHell syndicate hackers.

Perhaps the most famous black hat of today's generation is Anonymous which has gained media attention for its being a "digital Robin Hood," according to Thought Catalog. Unlike the previously mentioned black hats, Anonymous is not a single entity. It is a hacktivist group well known for its public appearances while wearing Guy Fawkes Masks.

The group has been famous for their hacking activities that targeted the Vatican, the FBI, the CIA, PayPal, Sony, Mastercard, Visa, Chinese, Israeli, Tunisian, and Ugandan governments. The group's primary goal is to attack internet censorship and control.

Lastly, perhaps the most feared and mysterious black hat to date is Astra. It is still undetermined whether he is a single individual or acting as a group. Astra is mainly concern with information related to weapons technology. He remains untraced by the various government agencies and has been wanted since 2002.

Watch the most dangerous hackers of all time below: