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Jordan Spieth plays a practice round prior to the U.S. Open at Oakmont Country Club in Oakmont, Pennsylvania, Monday, June 13, 2016. Sam Greenwood/Getty Images

The first round of the 2016 U.S. Open hasn't even started yet, but Oakmont Country Club, the Pittsburgh-area course that'll host the major golf tournament, is already living inside the players' heads.

"It's probably the hardest course in the world on any given day without messing with the setup," 2006 U.S. Open Champion Geoff Ogilvy told CNN.

Defending U.S. Open champion Jordan Spieth said he'd be fine shooting a 76, or six shots over par.

Or, as five-time major winner Phil Mickelson put it bluntly, Oakmont in 2016 "is the hardest golf course we've ever played."

The players are clearly anxious heading into the tournament tee-off Thursday, with good reason. The U.S. Open, historically speaking, is the most difficult of the four major golf championships. And while the event location shifts from year to year, Oakmont, founded in 1903, might just be the most difficult course that has ever hosted this most challenging tournament in golf.

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Jordan Spieth plays a bunker shot during a practice round prior to the U.S. Open at Oakmont Country Club in Oakmont, Pennsylvania, June 15, 2016. Rob Carr/Getty Images

The Most Challenging Tournament in Golf

The U.S. Open, scheduled to take place Thursday to Sunday, is generally the most difficult of the four major golf tournaments: the Masters, the British Open, the PGA Championship and the U.S. Open.

A post from veteran golf journalist Gary Van Sickle in 2014 called the tournament "the meanest, toughest, roughest, cruelest, stingiest SOB in major golf."

"The Open is the hardest major to score in, usually by a mile," he wrote.

The data backs that up. International Business Times tracked how many total strokes — or shots — it took the winner of each major from 1995 to 2015 to emerge victorious. On average it took U.S. Open winners 277.86 strokes to secure a victory across all four rounds, the most of any major. (Note to non-players: In golf you want to get through 18 holes in as few shots as possible.) The Masters followed closely at 277.14 strokes on average.

The Masters is always played at Augusta National in Georgia, a par-72 course, meaning the golfer is expected to get through 18 holes in 72 shots. The U.S. Open has a rotating cast of courses, including some like the 2016 host Oakmont, which is a par-70, yet it usually takes far more strokes to finish.

Conditions at U.S. Open courses are typically crafted to make it as hard as possible on the competitors. In the past 15 years, six major winners secured the victory finishing over par, and four of those wins came in a U.S. Open.

The last time Oakmont hosted the tournament, in 2007, it tore the players to pieces. The eventual winner, Angel Cabrera, prevailed as "the last man standing," wrote the New York Times. Cabrera finished five strokes over par, meaning five shots more than the course theoretically expects. That's tied for the worst winning score in at least 20 years. For comparison sake, the 2016 Masters winner finished five strokes under par and Tiger Woods stunned the golfing world in 1997 when he won the Masters by scoring 18 strokes under par.

And remember, the five-strokes-over-par finish was the best score of the four-round weekend. As the European Tour pointed out, there were 434 total rounds in 2007 and just six finished under par. The average round was, rather incredibly, almost 6 strokes over par.

The players simply couldn't get a grip in 2007. A group of top players that season selected by IBT shot an average round of 69.55 on the year, but those same players averaged a whopping 74.4 at the Western Pennsylvania course. And while some older courses get overpowered by strong players with modern technology, Oakmont has stood the test of time because it's difficult at every turn, making nearly every aspect of the game a challenge.

Don't Miss (Anywhere, or Any Time)

You better be accurate at Oakmont. It'll penalize you with thick rough grass and deep sand bunkers. Unfortunately, it's easy to miss at Oakmont.

Fairways, meaning the short grass where you want to land the ball on a tee shot on longer holes, are typically pretty skinny. At Oakmont in 2007 the field of golfers found the fairway just 52 percent of the time, compared with an average of about 64 percent for the PGA Tour players, as a group, for the season. Players at the '07 U.S. Open ended up on the green — where you putt and finish the hole — in the expected amount of shots (called a green in regulation) just 55 percent of the time. By comparison, the entirety of the PGA Tour hit greens in regulation at a rate of about 65 percent on the season.

And if a golfer does mess up a shot and ends up in the rough — the thick grass meant to punish a bad shot — they're in serious trouble. A number of pros have posted videos of balls simply disappearing in the rough, because it's so tall — about 5.5 to 6 inches or so — and thick.

Forget about hitting the ball far. Forget about hitting the ball at all. If a golfer hits a shot in the longest grass at Oakmont, the first challenge could simply be locating the ball.

 

Rough short of 17 green... Yeah, I'd say Oakmont is ready _ @usopengolf

A video posted by Justin Thomas (@justinthomas34) on

 

Back to the traditional US Open set up here at Oakmont.

A video posted by Graeme McDowell (@graeme_mcdowell) on

The Sand Is No Day at the Beach

Even if a player avoids the rough, the 2016 U.S. Open course is dotted with hundreds of bunkers. Two hundred and ten, to be exact. That's fewer than Wisconsin's Whistling Straits — a devious set of links imagined by legendary mad-scientist architect Pete Dye — but more than most elite courses. (August National, for instance, has 44.)

They're not fun bunkers either. Sand doesn't particularly trouble the best players in the world — at least not the way it perplexes your average weekend player. But No. 2-ranked Spieth has commented this week that Oakmont's bunkers are over-filled and soft, making them hard to navigate.

Greens Like Ice

If you do navigate the skinny fairways, the deep bunkers and the thick rough, you're still left to contend with the impossible greens. They're fast, meaning balls skate across them like ice. Miss a putt and the ball might roll 10 feet past the hole.

 

bit of slopes and fast green speed... @usopengolf #oakmont #⛳️ #_

A video posted by Byeong Hun An (@benan0917) on

Green speeds are measured using a stimpmeter, a simple instrument that measures how far a ball rolls when released from a 20-degree angle — the higher the number, the "faster" the green.

This whole system was invented back in 1935 by a guy named Edward Stimpson, who couldn't believe how fast the greens were at that year's U.S. Open, which was hosted by — you guessed it — Oakmont Country Club.

Oakmont's greens are expected to fall anywhere between 14 and 15 on the stimpmeter. They're largely considered some of the fastest, if not the fastest, in the world. An average elite-level green comes in at 10 to 11.

The only respite for the golfers might be rain. Rain softens grass, meaning greens are slower and balls are less likely to roll off fairways into rough.

Right now, forecasters predict storms early in the tournament. Usually that's a downer for golfers, but this week, the players might just be praying for rain.