CASTLE ROCK — From the tee box, it’s hard to tell if Jack Nicklaus lost a bet or his sense of humor.

The 10th hole at Castle Pines, home to this week’s BMW Championship, is the work of the Golden Bear by way of Genghis Khan, somehow splitting the difference between gorgeous and inhumane.

“Those (holes) are the ones you try to block out,” Steve Jones, the CU Buffs great and 1996 U.S. Open champion, told The Post recently, “and move forward in life.”

The hill off the tee drops like the Mariana Trench. The fairway slopes left to right. The rough to the left resembles a grassy version of the banked turns at Daytona International Speedway. The mighty pond to the right looks as if it’s carrying the green on its back, the way a mover might carry a small fridge up a flight of stairs.

“It wasn’t like a ‘risk-reward’ hole,” PGA veteran Mark Wiebe, a 20-year regular at The International at Castle Pines, added with a rueful chuckle. “There’s no reward. There’s a lot of risk.”

Miss port side, you’re into the trees. Miss starboard side, your ball’s sleeping with the fishes.

“It was just a really difficult par-4 teeing off from so high above,” recalled Rich Beem, the 2002 International champ. “And half the time the wind was blowing 15 to 20 miles per hour. … It was a hell of a way to start your day. But it was part of the fun challenge there.”

“There’s so much lurking”

No hole during Castle Pines’ International era — the PGA made the Nicklaus-designed course an annual tour stop from 1986 to 2006 — gave the best players in the world more challenges.

In the 21 years of The International, No. 10 rated as the most difficult hole 18 times. It wound up being the tourney’s most bogeyed hole 17 times.

“I was in a big playoff to go on to the next day, and I want to say maybe there were four or five or six of us,” Wiebe recalled. “And we went down to No. 10, and I had a 1-iron into that hole and the pin was over the water. And I’m thinking, ‘If I don’t clear this water, I’m gonna be sayonara; it’s too hard of a hole.’ I end up hitting the green, and I’m (remembering) that there were only a couple of us, or maybe three of us, who hit the green. I just remember 2-putting and being delighted at that time.”

The Golden Bear’s beast didn’t play favorites. It hated everybody equally. Usually.

“There’s just so much lurking,” Wiebe continued. “It’s probably one of the best views on the course. And one of the hardest holes to play.”

En route to winning the 2000 International, Ernie Els eagled the hole during the second round, opening with a perfectly placed 333-yard drive. The South African called it karma for five years earlier, when a final-round double bogey on the 10th helped to seal a runner-up finish at the ’95 event.

“That hole owes me,” Els told the Associated Press.

“It gets you in trouble”

The experts’ best advice? Aim for Castle Rock, poking out from the horizon, off the tee.

Then reach for the rosary beads.

“The ball is in the air for a long time and has a lot of time to go sideways,” PGA vet David Duval reflected. “I am curious to see what metals the golfers play (at BMW).”

That all-important opening shot must be struck high and into the wind, putting you at the mercy of the elements.

“If you didn’t have a good tee shot, it’s like, ‘OK, I’m just going to try to lay up and get (set up) for par,” Jones said. “A lot of times, you were so tempted to force it at altitude. ‘I can hit a 7-iron 190 (yards),’ and it gets you in trouble.”

What few gaps there are between plates of armor seem tiny and fickle. A 30-yard span of green on the left side of the fairway offers a friendly kick, sure. But the fates have to see you there first.

“There was a time where I actually hit it left (and) was behind a tree,” Drew Parr, head pro at Castle Pines, recalled. “And I thought, ‘OK, when I go to my second shot …’ This was that double scenario that can creep into your head pretty good.