It is a question thousands of golf fans are sure to be asking this week as they pack Royal Portrush for the first Irish Open in Northern Ireland since 1953.
How on earth did Rory McIlroy shoot 61 round this place? Even more amazingly, how on earth did he do it when he was just 16 years old?
Seven years on McIlroy is back as world number two and as a major winner as well, but he is also desperate to hit back from four missed cuts in his last five starts.
Although the last of those was his US Open title defence just two weeks ago, the Holywood star sounds upbeat as he said: "The first ever European Tour event to sell out is something that's obviously got a lot of people very excited and it's shaping up to be a great week. I've put 10 days of really good work in. My game feels good.
"It's just that if you are slightly off at a US Open it's so hard to get yourself into contention. In a way it couldn't be a better time to come back here and play Portrush. It brings back so many good memories and you can feed off that."
The main memory, of course, is that 61.
"I can basically remember every shot. I remember I missed a six-footer on the first for birdie - it could have been better! It was just one of those days where everything is on song. I turned three under, eagled 10, birdied 11, parred 12 and 13 and then birdied my way in. People call it 'in the zone'. Seven years ago - time goes pretty quickly."
And what a seven years for Irish golf. Dubliner Padraig Harrington achieved back-to-back Open Championship and back-to-back major wins in 2007 and 2008, then came Graeme McDowell's US Open victory two years ago - the first by any European since 1970.
However, McIlroy is trying a different attitude to previous Irish Opens - with good reason given his three most recent finishes in it were 34th, 35th and 50th.
"To be honest, the last couple of years I didn't quite enjoy the tag of home favourite. I just didn't feel very comfortable with it. This year I really want to embrace that. You look at so many people and when they've got a home advantage it is an advantage. It should be for not just me, but for the guys from here."