A trip to London invariably involves visits to Big Ben, Buckingham Palace, Westminster Abbey, the London Eye, and countless other iconic locations. One of the best ways to see all of these sights is by way of another London trademark: the black cab.

Not only is the black cab a convenient way to travel, but it is one of the surest ways to make sure you find what you are looking for. Before being allowed to operate the vehicle, the cabbie has been certified through one of the most comprehensive and difficult tests required of any profession.

There is much more to driving a cab than knowing how to safely operate a motor vehicle. London is filled with places of interest to tourists, businessmen, and residents. Being able to quickly and efficiently navigate to any of them without consulting a map or GPS is a must.

To ensure that all cab drivers know their way around the streets of London, each one must pass “The Knowledge.” It is considered one of the hardest tests in the world. To pass “The Knowledge,” London cab drivers are required to master no fewer than 320 basic routes, know and locate all of the 25,000 streets that are scattered within the basic routes, and be able to instantly plot a navigational route to any of the approximately 20,000 landmarks and places of public interest that are located within a six-mile radius of Charing Cross.

Those who master “The Knowledge” have been described as having had an atlas implanted in their brains. It takes the average person between 2 and 4 years to learn everything to successfully pass the test. You can generally spot those who are in preparation because they spend their days scurrying around London on mopeds, attempting to memorize the routes and sites. These aspiring cabbies are commonly known as “Knowledge Boys” or “Knowledge Girls.” In addition to memorizing all of the points of interest, they must demonstrate that they are “of good moral character.”

“The Knowledge” dates back to 1865 when cabbies had to navigate horse and buggy through the crowded London streets. Although the mode of transportation is different, the licensing and testing process has changed little since that time. An applicant can be certified only after being able to prove his or her ability to come up with an acceptable route, factoring in such things as traffic, construction, and other variables, and to do so without consulting a map, GPS, or consulting with a controller by radio.

Applicants show their mastery of London’s streets in a series of one-on-one oral examinations known as “appearances.” The typical applicant will need to pass twelve appearances and will take up to three years to do so.

In addition to tourist attractions, a cab driver must memorize streets, squares, clubs, hospitals, hotels, theatres, embassies, government, and public buildings, railway stations, police stations, courts, diplomatic buildings, important places of worship, cemeteries, crematoria, parks, and open spaces, sports and leisure centers, places of learning, restaurants and historic buildings. In short, everything within 800 meters (half a mile) of each of the 320 routes.

A route can be between any two significant points of interest in London. Upon being told the two points, the applicant must be able to immediately identify the fastest and most efficient route between them. For each route, applicants must be able to recite the names of the roads used, where the signals are, when they cross junctions, use roundabouts, make turns, and what is alongside them at each point.

Memorizing so much detailed information requires a lot of work. It also relies upon some biological factors. Studies suggest that preparing for and mastering “The Knowledge” can alter the hippocampus of trainee cab drivers. The hippocampus is the area of the brain used for spatial memory and navigation. Studies have shown that it tends to be larger in taxi drivers than in the general population.