Towering boulders and 100-degree heat hindered an extensive desert search Friday for a 17-year-old boy who disappeared while hiking in Joshua Tree National Monument.

"It's like a maze of boulders. You walk in amongst them, and they're 80 to 100 feet high around you," Supervising Park Ranger Cindy Von Halle said of the Jumbo Rocks region where Eric Sears disappeared Thursday. "It's a place where we don't like to lose people."

Sears, from Oceanside, and another 17-year-old boy whose name was not available had been camping at Jumbo Rocks Campground. They went for a hike, became separated and Sears disappeared, said Von Halle.

Park rangers were notified about 5 p.m. Thursday and a search began about 6 p.m.

On Friday, 50 to 65 searchers were assisted by search dogs, two helicopters and an expert tracker, officials said.

Von Halle said Friday evening that the search should resume at 6 a.m. today if Sears is not found before then.

"Somebody will be there all night," she said. "We've got people driving around all the roads in case he comes out onto a road somewhere."

Visitors can easily become lost among the rocks, said Von Halle, because trails often dead-end at scenic lookouts or rock-climbing areas.

Jumbo Rocks is about 4,000 feet above sea level, an elevation that helps limit daytime temperatures to 100 degrees - slightly cooler than the surrounding area.

"Twentynine Palms is about 103 here today. It has been 110 already this year, though," Von Halle said. "This time of the year, we encourage people to do their hiking early in the morning, or late in the afternoon."

Sears is 5 feet, 11 inches tall and weighs 150 pounds. He was wearing khaki shorts and a white shirt, but Von Halle said she did not know whether he was carrying any water, food or equipment v

Along with heat and dehydration, the greatest dangers facing lost hikers in Joshua Tree are falls, said Steve Bryant, a member of the Riverside Mountain Rescue Unit.

"People tend to climb a rock to see if they can find a highway, and it's easy to fall as you're trying to get down," he said.

Summer desert hikes are especially dangerous, said Bryant, who offered several tips.

"Hike at night with a flashlight so you don't get as dehydrated. Carry three times as much water as you think you can possibly drink. And wear light, reflective clothing," Bryant said.

For people who insist on hiking during the day, he suggests that they look for shade in canyons, behind large rocks and - in the high desert - behind or beneath vegetation.

Major searches occur at Joshua Tree National Park about once or twice a year, rangers say.

A five-day search in April 2003 led to the rescue of 60-year-old Marvin Matsumoto of San Jose, who was stuck in a hole among the rocks.

When found, Matsumoto was conscious but severely dehydrated and suffering from ankle and head injuries.

His first request: water.