One-of-a-kind escalator hauls orbs up the hill to waiting Gravity Park passengers.

Outdoor Gravity Park

Pigeon Forge, Tennessee

Even folks who have a peculiar grudge against gravity (such as the eccentric Roger Babson) would probably find something enjoyable at OGP -- Outdoor Gravity Park.

Balls roll down, then this Sisyphus pushes them back up.

Tucked into a grassy hollow on the outskirts of tourism capital Pigeon Forge, OGP taps the underutilized entertainment potential of one of nature's fundamental forces. Imagine several human-size hamster balls rolling down a hill with wet, shrieking people inside and you'll quickly understand its charm.

The activity, known as Zorbing, began in New Zealand in the 1990s as a way to "walk on water" in a big, clear, plastic bubble. In Pigeon Forge, the process has been turned inside-out: the water is now inside the ball -- or, rather, a ball inside the ball -- and acts as a lubricant for its human passengers as they slosh and slide, limbs flailing, down the hillside. Gravity does all the work.

Kaitlyn Lawson, the Park's marketing director and self-described "test dummy" showed us around. Kaitlyn guessed that she'd hamster-balled the hill "probably 500 times," once dressed as a Tyrannosaurus rex in tribute to the gyrospheres in Jurassic World (One of her dreams, she said, is to someday ride the hill in a ball filled with cooked spaghetti).

Shrieking, sloshing riders rumble past in their suspended inner orb.

Despite its natural appearance, the OGP hill has been carefully sculpted to provide controlled thrills to the widest possible audience (Minimum age is 5 and there is no maximum age; Kaitlyn said that the oldest rider she'd met was 98). "I had to walk in front of the bulldozer four, five, six times until I got the angles and curves I was looking for," said Chris Roberts, the engineer who created the Park. His professional training, he said, was not especially helpful. "Now I've got all kinds of procedures and manuals, but I had to learn how to do all this stuff through osmosis."

Lodge pin map reveals a lack of Zorbing west of the Mississippi.

The balls, officially OGOs (Outdoor Gravity Orbs), are made of bouncy, translucent plastic and cost $12,000 apiece. The inner, passenger sphere is suspended within the 11-foot-high OGO by hundreds of anchoring strings. It takes 122 hours just to hand-stitch one ball.

Orb-riders make their reservations in advance to reduce wait times. Once they arrive they sign a waiver, secure their valuables, use the changing rooms to put on their swimsuits, and then are driven to the top of the hill in a mini-bus. They have three downhill trail options, varying in degrees of velocity and bouncing. Once selected, the staff hoses 15 gallons of chlorinated water into the inner ball, the riders (up to three) dive in through an access valve, they're zippered inside, then are sent on their way. The spheres weigh hundreds of pounds with water and passengers, yet their smooth surface and single point of ground contact make them easy to roll. "I can't do a pushup, but I can launch one of these balls," said Kaitlyn.

Outdoor Gravity Park with descending orb on the right.

The orbs' progress down the hillside looks leisurely from the deck outside the Park lodge, but that is not the impression inside the balls, where the chaos is captured on handheld GoPro cameras that OGP happily rents to visitors (You can use your own smartphone, but you'd better seal it inside an extremely waterproof baggie). Without friction it's impossible to stand upright, and any bounce or bump sends you and the water sliding up the inner wall, legs in the air. Riding downhill in an OGO has been compared to being in a washing machine and on a roller coaster at the same time. Even from outside the sealed balls, you can hear the squealing passengers. "The muffled screams are always so much fun," said Kaitlyn.

Orb-rider exits in a birthing burst of water.

When the orb stops at the base of the hill, the valve is opened and the rider tumbles out in a gush of water. Its resemblance to the moment of childbirth is frequently commented on by laughing onlookers.

Kaitlyn noted that the average waterslide ride takes 30 seconds, while a roll down the OGP hill lasts three times as long. Although she described herself as "the kind of person who could ride a roller coaster 12 times in a row," Kaitlyn said that she could only handle OGP's Orange Trail -- the most maniacal -- once a day.

Several factors have limited Zorbing in America to this one spot, including startup costs, bewildered insurance companies, local regulations (some states are surprisingly uptight about people in giant balls), and a proper hillside: not too shallow, not too steep. Wind is a surprising adversary. Gusts can potentially blow the orbs into places that they shouldn't go, and downdrafts can unexpectedly boost their velocity -- a concern for an attraction where the physics of orb momentum have been carefully calculated. Kaitlyn said that weather closes OGP roughly four days a year.

Another human hamster ball approaches the home stretch.

Most surprising to us was that this outdoor, swimsuit-wearing attraction is open year-round. Solar radiation warms the air inside the ball to twice the outside temperature (a good reason to have water in the ball in July). Wintertime orb-riders wait their turn in the heated lodge, transfer to the heated mini-bus, then ride the balls in water that's been pre-heated to a jacuzzi-like 95 degrees. "When people get out, steam is coming off of them," said Chris. Kaitlyn said that adrenaline also helps to keep the riders warm. "Sometimes it's crazy raining, sometimes it's 30 degrees, and people are, like, 'I still want to go!' And you're, like, 'Cool, let's do it!'"

In May and September an additional option is available: the Running of the Balls. One time, said Kaitlyn, a groom paid to ride the ball over every member of his wedding party, but usually the runners are a group of people who pay to try to outrace an orb down the hillside (The money is donated to local charities). "I've been rolled over I don't know how many times," said Kaitlyn. "It feels like a deep tissue massage." Finding someone to ride in the ball at such moments, Kaitlyn said, is easy: all you have to do is to ask the assembled crowd, "Hey, does somebody want to run people over?" There's always a volunteer.