On the morning of Friday, October 21, 2016, America woke up to find that the internet wasn't available for much of the Eastern Seaboard. Dyn, a company that provides Domain Name System (DNS) services — the web's directory of addresses, basically — to much of the internet had been taken down by a massive Distributed Denial of Service attack from the Mirai botnet.
Mirai was originally created by three men, Paras Jha, Josiah White, and Dalton Norman, who ran a company that sold defence mechanisms to DDoS attacks. In order to drum up business, they created Mirai to launch a DDoS attack on a French web hosting firm, OVH. They were hoping that companies who hosted servers for the millions of people who play the online game Minecraft would pay them to make sure their servers never fell victim to a DDoS.
Their experiment was too successful. Afraid of the monster they created, the trio published the Mirai code online in hopes of disguising their role in creating it. The code was then used by other hackers to target Dyn.
It was the first serious indicator that a hostile third party has the ability to send the world's foremost military power back to the pre-1990s era of telecommunications.