Imagine surfing, but picture doing it downhill on a hard, smooth surface at speeds up to 55 miles per hour. In the dark. With a bunch of other people flying down the hill around you. In a nutshell, that's downhill longboarding in the Valley, where rides and timed races take place every month during the full moon at McDowell Mountain Ranch.Longboarding was developed on the West Coast in the 1950s and requires the use of a wheeled board that is generally up to 59 inches long. The downhill aspect, in which riders fly downhill as fast as they can while trying to maintain control of the board, has a large following in Phoenix but has stayed pretty quiet while growing over the past two years. Anywhere from a dozen to 25 people participate in the full-moon rides, as well as at downhill longboarding events at South Mountain, but the events are so under wraps that they're almost impossible to catch. (In fact, they eluded the Best Of team all summer!) The full-moon rides at McDowell Mountain Ranch are organized by former NAU student Ryan Chopko, who announces monthly events only to members of a longboarding group on Facebook. He says the moonlit rides are "pretty intense.""It's dark. There's not much light, except the light from the full moon. And we move down the hill really, really fast," Chopko says. Generally, boarders meet at a location near McDowell Mountain Ranch around 9:30 p.m. Riders are shuttled to the top of a hill and dropped off. Speeds are clocked by timekeepers at the top and bottom of the hill, and if everybody's pitched in some money, winners can get prizes. (Chopko generally asks for a $5 buy-in per boarder.)You can only keep a good secret for so long. What started as an underground event is quickly moving into the mainstream. "Our full moon event is . . . becoming so well known it almost doesn't require advertisement," Chopko says. "Which is so crazy to me, because two years ago, it was something so small and private. No one knew what the heck it was."