The Underground is a newspaper made by the original members of the Young Tribulation Force.
At the end of book 5 Vicki brings up the idea of starting an underground newspaper.
The Underground, as named by Judd is distributed it throughout Judd and Vicki`s school.
Sometime after the kids decide to make the newspaper into a website since they are no longer in school and may have a hard time handing out the copies. They make the site and throughout the years the young trib force makes it grow. Soon it is on the WEbsite Not To Go On list by the Gc, Since they thin htese sites are dangerous. That however olny makes more people go on since they are now curious what it is all about. In book 39 Chang tells Judd, VIcki, And Lionel, that the GC has made changes to the website. That means that they have finally broken into the site and more importantly the have an address on their friends.