How hard could learning a map of a city be? In London, earning the credentials to drive one of the city’s iconic cabs is equivalent to earning a university degree. It’s so advanced, in fact, that being able to navigate the streets isn’t just considered knowledge, but is formally called “The Knowledge.” The way London’s taxi drivers talk about it, it seems a little like getting a black belt in karate while becoming an Eagle Scout while vying for admission to Mensa.

The reason why is London’s curious urban design, a squirrely mix of streets that were designed over centuries rather than by a one-time urban design grid that you might find in New York or Washington DC. There’s no pattern to learn in London, or a system of mnemonics to remember the order of roads. You simply have to learn every street in the city. And before you can legally drive a taxi, you have to prove to a group of city officials that you can, without fail, navigate between any two points. During the tests, aspiring drivers have to dictate the most efficient route and recall landmarks they’ll pass on the way. The people who are very good at it—and let’s be honest, more than 90 percent are men—can master the system in two years. Most people take four or longer.

It’s a fun tourist novelty to know that the person driving you has a very detailed spatial map of the city in his head. But for about a decade, a group of researchers at the University College of London have looked into the effect that memorizing such a disorganized system has on your brain. The part of the brain that navigates spatial intelligence is called the hippocampus, a pair of two chestnut sized masses toward the back of your head. The researchers found that London cab drivers have uniquely bigger hippocampi than almost anyone else.

We asked a few London cabbies about this in hopes they could help us understood how their brains worked.

“Oh yeah mate, it’s called the hippocampus,” one cabbie named Simon told us. “Most people don’t use it because of the simplicity of navigating most other places and because of maps and GPS. But with London there’s really no other way.”

What’s it like to map something very complex in your brain, we asked?

“Well, right when the person asks where to go, it’s like an explosion in your brain. You see it instantly.”

An explosion in the brain is a pretty vivid image to understand just how someone’s mind works. Yet it rings true. Each time we got into a cab and stated an obscure street name or small neighborhood, the driver didn’t even respond. He just started driving, seeming to know immediately which streets to take, and what the most direct route would be.

The downside to having a big hippocampus is that when cabbies retire and stop using their spatial mapping so regularly, the hippocampus actually starts to shrink back to normal. It’s like a muscle that shrinks if you don’t use it. What’s more, memorizing such a detailed map of a sprawling city actually took up the place of other grey matter. Researchers found that cabbies were worse at remembering things based on visual information and had worse short term memories. There is, after all, only so much real estate in one’s head.