Drug smuggling submarines. There was the court case of El Chapo and his IT guy testified about how he requested everyone's phones to be bugged. Also a secure comm network built for his organization, that then ended up bringing him down, because the IT guy folded (the FBI made a deal with him), etc.

Also you could learn a lot about tunnel building, and tunnel detection.

The crime syndicate scenario would be actually better because unlike advertisers, they actually provide (illegal) products such as drugs that have demand. I can give you examples of people happily buying drugs, but I’m not sure I could ever find someone who’d be willing to look at ads/spam voluntarily, let alone pay to do so.