The load roar made it easy to figure out that he holed the 81-yard shot for his third eagle of the round -- and second on a par-4 hole.
Playing in perfect scoring conditions on Angus Glen's North Course, Mahan also holed out from 189 yards on No. 18 -- the ninth hole of his morning round -- and made a more routine eagle with a 10-foot putt on the par-5 11th. He finished with a 9-under 62 to match the tournament record and take a two-stroke lead.
'Golf's a funny game,' Mahan said. 'Sometimes you have it and sometimes you don't. Sometimes you do everything you're supposed to do and it doesn't work out. You have to be patient and you've just got to keep working.'
Three other players have shot 62s in the Canadian Open, all at Glen Abbey. Leonard Thompson set the mark in 1981 and Andy Bean matched it in 1983, both when Glen Abbey played to a par of 71. Greg Norman did it in 1986, when par was 72.
'That is pretty special,' said Mahan, the only one to do it in the first round. 'Especially, a tournament like this that's been around forever.'
The 25-year-old former Oklahoma State star won the Travelers Championship last month in Connecticut for his first PGA TOUR title, then tied for eighth in the AT&T National and had a sixth-place tie Sunday in the British Open. In the Travelers, he also opened with a 62 and went on to beat Jay Williamson in a playoff.
'My mind is kind of catching up with my game,' Mahan said. 'I've been swinging great. I've been driving it really, really well and hitting my irons solid.'
Australia's Steve Allan was second after a 64, and Canadian David Hearn was another stroke back along with Tripp Isenhour. Mark Calcavecchia, the 2005 champion at Shaughnessy in Vancouver, shot a 67 in the breezy afternoon session. Jim Furyk, the winner last year at Hamilton Golf and Country Club, had a 69, also in the afternoon.
Mike Weir shot an even-par 71, putting him well back in his bid to become the first Canadian winner since Pat Fletcher in 1954. Vijay Singh, paired with Weir, opened with a 68. Singh beat Weir in a playoff in 2004 at Glen Abbey.
'Wasn't a great day, but made some nice putts and got the ball around OK,' Weir said. 'Just kind of strange. Just had a couple of lies just barely in that first cut that I felt like I could have controlled a little better.'
Mahan was 4 under after three holes. He made a 3 1/2 -foot birdie putt on the par-4 10th, holed the 10-footer for eagle on the par-5 11th and made a 14-foot birdie putt on the par-4 12th. He reached 5 under with a birdie on the par-5 15th, dropped a stroke on the par-3 17th, then holed his 6-iron approach shot on 18 for his second eagle.
'It was just about as perfect a shot as I could hit during a round of play,' Mahan said. 'The pitch mark was right next to the hole.'
On his second nine, he bogeyed the par-4 second to fall back to 5 under, but made a 20-foot birdie putt on the par-3 fourth and a 7-footer on the par-4 sixth to reach 7 under. He capped his first three-eagle round with the 2 on the 372-yard ninth.
'I didn't see it,' Mahan said. 'Set kind of a flat sand wedge in there, took a little spin off and it spun right back in the hole.'
Allan, the 33-year-old Australian who won the 1998 German Open and 2002 Australian Open, had eight birdies and a bogey in his morning round. He tied for 13th last week in Milwaukee, his best finish in 13 starts this year.
'It was perfect this morning,' Allan said. 'There was hardly a breath of wind. The greens were soft and the pins were accessible. ... I think it's going to play pretty easy unless the greens firm up, because they're really soft out there today.'
Hearn, from Brantford, was the top Canadian last year, tying for 20th. He's making his first PGA TOUR start of the season after playing 16 Nationwide Tour events.
'I definitely feel like my game's at a level where I can do well at these events,' Hearn said. 'I'm going to do everything I can to do that.'