Marianas web is the deepest, darkest part of the internet. it cannot be accessed using a regular browser or regular methods.
The DarkNet is made for anonymity; the sharing of information without fear of detection or prosecution. Due to its nature this attract many people, both bad and good. some to speak freely of political jurisdiction and others to engage in all manners of illegal activity.
Like the 'normal' web, Marianas web has a range of topics, but mainly illegal activity such as cannibalism, child porn, sales of weapons, hitmen, human trafficking, terrorism and more.
The Web is made up of the 'Surface Web' and the 'Invisible Web', think of the internet like an Iceberg; only some of it can be seen.
The 'Surface Web' is the content we can all see, its index by search engines or its link to by other web pages.
The 'Invisible Web' is just that, invisible, its not In Google, no sites link to it, so its invisible. The Invisible web is then split into the 'DeepNet' 'DarkNet' or w/e you want to call it, this is webpages that cannot be accessed with a normal browser, there are measured to keep you out.
And thats exactly what Marianas Web is, its the Deepest part of the Web, where people dont want you to go.