Well 15% monthly woud be ~535% annual and at that point you're well past "highly unlikely" and into "I'm printing money in my 20k sqft basement" territory.

We had a big Ponzi that exploded last month in Argentina (and extended to a few nearby countries). They promised a monthly interest from 7.5% to 20% (and IIRC they has even better offers for special "investors"). For example see https://www.ambito.com/informacion-general/generacion-zoe/le... autotranslation https://www-ambito-com.translate.goog/informacion-general/ge...