Only sky shows through the windshield as Mick Stine powers his lime-green AMC Eagle up a nearly-vertical dirt ridge.

The engine growls and whines as the Eagle climbs. Its 36-inch tires spin in the loose soil, throwing dust and small rocks into the air.

On the ground below, a crowd of a dozen or so people, many carrying cameras and camcorders, watch the green beast seemingly defy the laws of physics as it clings to the sheer incline.

Finally, the car lurches up and over the top.

"Child's play," Stine shrugs, as he steers the Eagle to the safety of flat ground.

Stine and his friends are engaged in the first-ever Poison Spider Off-Road Challenge, a modified version of the playground game "HORSE." Only instead of competing with a basketball, they're using rock crawlers: four-wheel drive vehicles designed to get their drivers over boulders, through ravines and up steep hillsides.

The rules are simple: the lead driver chooses an obstacle to drive over or through. If he's successful, the next driver must complete the same obstacle or receive a letter. You're out when you spell "crawl."

The challenge has drawn 12 competitors to Poison Spider Off-Road Park on this hot and dusty Saturday afternoon. Some of their vehicles are stock Jeeps and trucks, no different than one that just drove off the lot. Other rock crawlers are highly modified rigs, with specialized suspensions and tires, and a body wrapped up in a several-hundred pound exo-cage.

Many motor sports, from midget cars to Formula One, are based on speed. Rock crawlers, on the other hand, rely on technique and finding the right approach to overcome an obstacle.

Crawling is an apt description as drivers move slow enough that all four wheel remain on the ground. Spinning tires means the loss of precious traction.

"You have to have some patience," explains Tim Haid, president of the off-road club Backcountry Crawlers, which, along with Casper off-road shop Evolution 4x4, put on the challenge.

A good driver must read the trail to succeed. Haid likens it to a golfer reading the green.

"You really have to think about where your tires are," he says.

Stine and two other drivers are competing in the extreme class - which features vehicles with tires of at least 36 inches. Over several hours, they challenge various obstacles in the vast park. Some involve driving up and down slopes that would be nearly impossible for a hiker to manage. Others are similar to a giant slalom ski race, except that instead of gates and skiers, there are rock crawlers navigating through trees in a narrow ravine.

"This one is kind of spooky," Stine says as he guides his Eagle through a ravine. Branches slap the car's body as it passes under trees. The engine revs as the Eagle goes up a ridge and, with a noticeable bump, down the other side.

"There goes a speaker," Stines says, as something slides around in the back.

Stine, who co-owns Evolution 4x4, says he got into rock crawling in 2001, after getting bored with four-wheeling through mud.

"I always liked driving," he says. "It's just a form of technical driving. It requires skill. You got to know your rig a lot."

Before Stine tackles the next obstacle, his passenger asks him whether he's ever rolled his rig.

"Not yet," he says. "But I'm not opposed to the idea."

Competing with Stine in the extreme class is Travis Dickau, a Glendo man who brought along a red GMC Jimmy truck. He says he enjoys rock crawling for the comradery.

"And the rush," he adds.

That's evident as he powers his truck up a sheer, roughly 50-foot slope. The engine belches and the tires spin, but the truck seemingly can't overcome gravity. Finally, Dickau hits the accelerator and the Jimmy finally surges up and over the edge.

"It gets the heart pumping," he says.

Reach Joshua Wolfson at (307) 266-0582 or at josh.wolfson@casperstartribune.net.