National Geographic:

The Knowledge of London is a real-time, street-level test of memorization skills so intense that it physically alters the brains of those who pass it.

To qualify for that elusive green badge, you need to learn by heart all 320 sample runs that are listed in the Blue Book, the would-be cabbie’s bible. You will also have to commit to memory the 25,000 streets, roads, avenues, courts, lanes, crescents, places, mews, yards, hills, and alleys that lie within a six-mile radius of Charing Cross.

I am severely directionally challenged and couldn’t complete The Knowledge on pain of death but reading about the process they have to go through is fascinating.