The Knowledge: https://www.nytimes.com/2014/11/10/t-magazine/london-taxi-te...

"It has been called the hardest test, of any kind, in the world. Its rigors have been likened to those required to earn a degree in law or medicine. It is without question a unique intellectual, psychological and physical ordeal, demanding unnumbered thousands of hours of immersive study, as would-be cabbies undertake the task of committing to memory the entirety of London, and demonstrating that mastery through a progressively more difficult sequence of oral examinations — a process which, on average, takes four years to complete, and for some, much longer than that."

It's called "the knowledge" and not only do they need to know 20,000 landmarks (things like The Royal Institution of British Architects building, which is not a very touristy landmark at all) they also need to know 20,000 streets and the best route between them. On their test they have to describe a route across London between 2 landmarks and name every road, roundabout, where they should be in the road when there are multiple lanes, one way restrictions and it has to be perfect. It's an incredibly difficult test to pass.