What is the underground app?

Excuse me, could you please elaborate on the concept of the "underground app"? I'm intrigued to understand what it entails and its purpose within the realm of cryptocurrency and finance. Are these apps operating outside of traditional regulatory frameworks? How do they function, and what kind of services do they offer that might be of interest to those in our industry? Furthermore, are there any risks or considerations one should be aware of when dealing with such apps? I'd appreciate your insights on this matter.