Welcome to Harry Potter Theory. Today we’re discussing Albus Dumbledore, and trying to unravel one of his dirtiest darkest secrets- how he got his scar of the London Underground.
As early as the Philosopher’s Stone (in the first chapter, no less), we hear Dumbledore mention a certain scar in a conversation with McGonagall. But it’s not the scar that you probably thought of, Harry’s scar of legend, it’s actually one of Dumbledore’s OWN scars. 
If the first place your mind went to was emotional scars then that would make perfect sense, as Dumbledore, who lived quite the tumultuous life, would be riddled with them. 
But in this instance we’re talking about a real bonafide scar. 
The following is Dumbledore’s response to McGonagall’s query about whether or not he could remove Harry’s scar:
“Even if I could, I wouldn’t. Scars can come in handy. I have one myself above my left knee that is a perfect map of the London Underground."
For those unaware, the London Underground is a public transport network of underground railways in London, also called The Tube. In this case, Dumbledore is referring to the Tube Map, which is a map used by travellers on the underground. However, it’s not simply an overlay of the railway lines onto a map of London; it’s a simplified version of lines to make it easier to follow. 
Some may have interpreted this little quip as some sort of joke, but as it happens, JK Rowling has since confirmed that this scar on Dumbledore’s left knee was very much real. The problem, however, is that she has never revealed HOW he got it, even avoiding the answer at a public Q&A 7 years after the first book’s release.
“How did Dumbledore get his scar of the London Underground?”
“You may find out one day. I am very fond of that scar.”
And now, 25 years after the release of the first book, this question, like so many others, remains UNANSWERED.
Today, we’re going to be diving in and figuring out the cause of Dumbledore’s famous scar.