Courtsiding, or the simple practice of sitting on the sidelines of tennis matches and relaying scores via phone to hungry gamblers, can be an incredibly lucrative business. For instance, Sporting Data Ltd, a UK-based private betting company, managed to rake in millions on the speedy fingertips of its young employees. The work, to hear the young men tell it, is good, too. You get to travel around the world following the major tennis tours, watch exciting matches, and make money just enjoying the game. But to call the business controversial would be to put it lightly, as it represents a shadow industry tennis officials are trying to shut down. Just ask Daniel Dobson: back in January, the 22-year-old courtsider was arrested at the Australian Open for violating the "integrity" of the sport. Since then, Sporting Data has shut down its courtsiding activities, and is speaking openly about the practice. Find out more about the tennis betting underworld at FiveThirtyEight.