The internet contains at least 4.7 billion websites that have been indexed by search engines, according to one Dutch researcher. That huge number barely scratches the surface of what's really out there, however. The rest is known as the deep web, which is 400 to 500 times larger than the surface internet, according tosome estimates. Most of that is made up of innocent content, such as emails, social media profiles, subscription sites, and anything that you need to fill out a form to access. But because the deep web is hidden from search engines, some people use it for more nefarious purposes.

The dark web is the subset of the deep web that's known as a haven for criminal activity. Ranging from drug transactions like those of the now shut-down Silk Road to resources for hitmen, terrorists, and pedophiles, the dark web's illicit marketplaces generate more than $500,000 per day. That's made possible by its near total anonymity, thanks to the lack of DNS and IP addresses that usually make websites identifiable. Users can only access sites on the dark web through special software that encrypts their activity and routes them through random nodes to get to where they're going, making it harder for anyone to track them. Still, even accessing the dark web can be enough to set off red flags at the FBI, and it's easy to make mistakes that can tip off law enforcement.