One of the defining moments in X Games history was in 1999, when Tony Hawk became the first skateboarder to successfully land a 900-degree rotation above the lip of a vert ramp.

That's 2 1/2 spins while airborne, and Hawk nailed the landing on his 12th try.

Fast forward to 2010 and to a relatively new contraption called the Mega Ramp, which has delivered vert skating to daunting new heights.

Skateboarders must speed 70 feet down one ramp, soar and perform a trick across a 70-foot gap to another downslope, which leads to a quarterpipe wall that sends them more than 45 feet above ground, where they perform a second trick.

Few people are brave enough to ride the Mega Ramp, let alone try a 900 above the quarterpipe wall, but that's the expected theme during tonight's Skateboard Big-Air final on the first day of X Games competition in Los Angeles.

"Obviously with Jake trying it last year he kind of opened the box a little bit," said Bob Burnquist, two-time Big-Air gold medalist, in reference to a failed 900 attempt by Jake Brown during last year's final.

"But at the same time it's pretty high. It's a whole different ballgame. It's different than regular vert skating -- it's straight hairball. You have to just go for it and try to keep yourself controlled and get out of it safely."

Burnquist has a Mega Ramp in the backyard of his home in Vista, Calif., and top riders have been practicing there, including Brown, who won the gold medal last year on a different run with a different trick.

Burnquist and Brown have spun 900s above the quarterpipe wall, but have always had to bail free of their boards and slide down the wall. The trick is that scary and difficult, and slamming awkwardly onto the lip and breaking bones, or free-falling to the flat surface below the ramp are among the consequences of holding on too long without being in perfect position.

Nobody is as painfully aware of Mega Ramp consequences as Brown, who in 2007 fell to the flat from 47 feet and was hospitalized with serious injuries after losing control before he could even begin a trick above the quarterpipe wall.

His landing was so violent that both shoes flew from his feet.

Brown and Burnquist, though, feel relatively comfortable performing 720s above the quarterpipe, so the next step level of progression, in terms of spinning, is the 900.

"It has not happened yet, not even in practice," Burnquist said. "I've been spinning them but I haven't really put one down because I haven't really wanted it yet, deep down. I've kind of been waiting for the right time and the right moment, and I know that Jake has been taking about the same thing so we'll see how it goes.

"It's possible, but the risks are pretty high."

Photo of Bob Burnquist competing in the X Games Big Air by Getty Images/Christian Pondella