The deep web

the deep web also the dark web (noun)

A part of the Internet that cannot be accessed with conventional browsers or search tools, often used for illegal activity.

'The underbelly of the Internet, known as the deep web, is 500 times the size of what we call the surface web – that which can be searched and indexed by the likes of Google, Yahoo! and Bing.'  
SC Magazine UK 18th July 2014 

'A senior RAF airman who served in Afghanistan has been jailed for making … weapons and then selling them on the dark web.'  

The Telegraph UK 2nd July 2014

             

It seems that the Internet is a bit like the sea – there's the frothy bit on the surface that everyone sees and routinely uses in daily life, but below it lies a gigantic chasm with hidden depths, only explored and utilized by a handful of specialists. It's this virtual underworld which is now sometimes described as the deep web or the dark web, and it often has rather sinister associations.

The exact proportions of the deep web haven't been established, but it's likely to be many hundreds of times bigger than the surface web, and contain a massive repository of data that can't be located by a simple Google search.

As more and more people across the world access the Internet, they're in fact finding less and less of the data that's actually stored online. The web as we know it can be viewed with standard web browsers and search engines, but in the same way that just the tip of an iceberg can be seen by observers, these tools only allow us access to a mere fraction of the web in its entirety. Buried below the web we know and love, which is often correspondingly characterized as the surface web, lies the deep (or dark) web, also known as the hidden/invisible web, or the undernet.
Source: Macmillan Buzz word