The green itself - 6,000 square feet of agony, wherever the pin placement.
"Just got on the wrong side," Rose said of his shot Saturday, which resulted in a bogey, followed by a bogey on 18, leaving him 2 behind 54-hole leader Phil Mickelson. "The pin was cut right between two little knobs . . . the ball got thrown off the green."
Then the pain really began. Rose knew the fate of his next shot. His pitch got halfway to the hole, still 15 feet short.
"Just trickled off a swale into that really terrible lie - you're really hoping you make contact," Rose said.
To review: Rose thought he hit a perfect first shot. And ended up hoping he could make contact on his second.
He did, barely.
"I tried to hold down a sand wedge just to try and get as much of the ball as I could with the toe of the club and didn't get any ball," Rose said. "So that's tough to take."
Just-misses around the traps brought even bigger trouble. There was Zach Johnson taking an unplayable lie from the heather above a right greenside bunker. First he dropped back into it, but the drop was unplayable, so he took a line-of-sight drop away.
On Friday Graeme McDowell hit into the same stuff, was able to play it, but had to lurch out of the way as his shot almost hit him on its way back into the sand.
Early Friday, Tiger Woods didn't think his tee shot was too bad.
"If it [lands] in the bunker, it's an easy bunker shot," Woods said.
It landed in a little tongue of heather that dipped into the bunker, and stayed there. Woods had to test a couple of off-kilter stances before hitting his pitch.
"I hit a good pitch," Woods said, "and the wind killed it."
It hit just off the green, six inches short of perfection, and stayed there. Woods threw his head back. Bogey.
The hole can play anywhere from 206 to 254 yards.
"They moved the tee up but they put the pin in a place that's virtually impossible to get to," Rory McIlroy said of the second-round 17th. "I felt like I hit a career shot and hit it to about 60 feet."
The 17th is just the last of Merion's devious par-3s. The 13th is the break from pain, the wedge shot, playing slightly (but only slightly) under par, but the third, ninth, and 17th are all monsters, averaging 3.4 strokes per golfer through two rounds, with the 17th averaging 3.35.
"During the practice rounds, they didn't even try to hit out of the heather," said 17th-hole marshal Mike Shannon. "They had us just shag it back to them."
If Mickelson wins the Open, he should look back at 17 on Saturday as a key moment. With everybody in the field praying for par, Mickelson drained a birdie that must have felt like an eagle, after a big 4-iron that tracked to about 12 feet. When he drained it, the roar from the crowd could be easily heard over half a mile away.
That's the theater that awaits on Sunday, even before the killer par-4 18th, Ben Hogan's hole, finishes things off.
Contact Mike Jensen at firstname.lastname@example.org. Follow @jensenoffcampus on Twitter.