In November 2023, Clarivate Plc announced that it had excluded the entire field of mathematics from the most recent edition of its influential list of authors of highly cited papers because of massive citation manipulation, which in return influences the so-called "Shanghai ranking" of top universities (or those claiming to be top). While most mathematicians would probably not care, the exclusion is in fact the tip of the iceberg of a parallel universe of predatory and mega-journals whose main purpose is to offer publishing opportunities for whoever is willing to pay the right price. I will explain how the system works, why we should care, and what measures we can all take against.