At 15, Scott Senatore of Fort Lauderdale has literally turned the world of skateboarding upside down.

Scott has designed and built a large translucent cylinder -- "the Dome" -- in which skateboarders can make a complete 360-degree circle. For a split split second they actually skate upside-down.

"It's a rush, a real," beams Senatore. "Your feet are planted on the ceiling. You're defying gravity."

Eighteen felt wide, 27 feet long and weighing 15,000 pounds, the cylinder most closely resembles a beached celluloid whale.

The dome, named Turningpoint Madness, is not for your average, run-of-the-sidewalk skateboarder.

In fact, it's strictly for professionals like Senatore. Some of them can complete up to six consecutive 360-degree revolutions -- something unheard of in a sport where sloped swimming pool walls and the insides of unused storm drains provided the setting for most previous daredevil stunting.

Senatore's dome began to take shape as he was daydreaming during his ninth-grade drafting glass in his native Malibu, Calif. That's when he first thought of a design for a surface that would give the skateboarder a chance to capitalize on gravity instead of fighting it.

The result was a circular "dome" open at one end and rounded off at the other. Aerodynamically, it would allow a skateborade to build enough momentum to streak downward with enough centrifugal force to safely complete a 360-degree turn.

And for a little faster speed, why not use gravity's pull even more by using a hydraulic lift to raise the front end of the dome about 15 feet? That would create a 30-degree angle and give the skateboarder just the lift needed to complete the potentially hazardous full circle.

It took Senatore and three adults, including an engineer, thousands of hours and $65,000 in materials to turn the design into a skating dome prototype.

The moveable structure is made of a clear plastic called lexan. Sixty-six 4-by-4-feet sheets of the half-inch thick plastic are held together by more than 10,000 screws. Aluminum griders encircle the dome and hold the plastic sheets in place.

"It is really a trip on your mind," said Senatore.

Counting the seven-foot platform on which it rests and the 15 feet added by the hydraulic lift, the top of the dome is nearly 40 feet in the air.

"On concrete you can see your transistion [from where you were to where you are going], but on the dome's clear glass, it's like you are flying," says Senatore.

An added touch is that the dome is attached to the platform in a way that enables it to revolve sowly on a horizontal [sideways] axis. This gives the skateboarder a feeling of riding a wave, Senatore explains.

"When you surf, you are moving on a moving surface. When you skateboard you normally are moving on a stationary surface. Now we've added a little more excitement to skateboarding made it like surfing on land. A lot of good skaters have trouble adjusting to that lateral movement [of the dome]. It's like an invisible wave is slapping you from the side."

Senatore recommends that all skateboaders warm up by stretching prior to skating. His advice is to "bend down, stretch your legs and arms for a few minutes before even getting on the board. And then take it easy in the beginning.

"You should always try the easy part of the run first, don't just drop into the hard part. You have to get the feel of your board again."

And if you ever become proficient enough to try skating inside a dome such as this, you should be sure to skate directly over the aluminum griders that encircle the dome and anchor the lexan surface.

"The aluminum gives you a little more solid push so that you can develop your speed. But if you're doing a trick or make a turn, you should do that off the aluminum rails and on the sheets of plastic on the dome because the lexan is a lot softer and it grips better."