Along some of Arizona's most beautiful hiking trails, piles of trash left behind by illegal border crossers lie under trees.
Some trails are hard to follow because illegal immigrants have carved paths of their own.
And encounters with Border Patrol agents on foot, horse or ATV are common.
Most of Arizona's borderlands remain open to hikers, birders, campers and hunters, but they're not as secluded or pristine as they used to be.
"There are so many things that take away from your opportunity for solitude and the outdoors experience," said Matt Skroch, a longtime hiker and executive director of the Arizona Wilderness Coalition.
Most run-ins with Border Patrol agents are quick and friendly, said Steve Singkofer, a hiker and former president of the Southern Arizona Hiking Club. Like most outdoor enthusiasts, he respects and understands the need for border-enforcement measures.
But the agents' omnipresence - made necessary after Arizona became a key smuggling route in the late 1990s - changed the dynamic of outdoor recreation in Southern Arizona.
"They don't mean to be intrusive," Singkofer said, "but by being there, they are."
Sometimes that can lead to startling encounters. Hikers have found themselves surrounded by armed agents after accidentally tripping ground sensors in the Huachuca Mountains, Skroch said.
And then there is fear of armed smugglers. Bill Bens, a member of the Quail Creek hiking club of Sahuarita, no longer hikes in the mountains and canyons just west of Interstate 19 because of reported shootings by suspected drug smugglers, including the gunbattle last December that killed Border Patrol Agent Brian Terry.
"I'm just not sure about it," Bens said. "But there's a lot of other great areas to hike."
Al Aguilar, a 76-year-old veteran who has been hunting along the border since he was a child, says that on hunting trips his sons and grandsons now take turns on two-hour night shifts to guard their camp.
There is a perception - and some reality - of danger and "parsing perception from reality is difficult on the border," Skroch said.
Some longtime hikers and birders don't worry about dangerous encounters with smugglers because they know that people trying to sneak drugs and people into the country want to avoid them. And many say they feel safer today than five or 10 years ago, when people-smuggling peaked and the Border Patrol was only half the size it is today.
The buildup of border enforcement has also led to upgraded dirt roads used by birders, and means agents are nearby when people need help.
Professional bird-watching guide John Yerger of Tucson's Adventure Birding Company was in Sycamore Canyon when a hiking companion fell ill with food poisoning. Yerger hiked out, planning to call paramedics as soon as he got cellphone service.
But before that he found Border Patrol agents who went into the canyon with a medic to help his friend out.
Increased border security, Yerger said, "comes with its blessings and its curses."
Contact reporter Brady McCombs at 573-4213 or firstname.lastname@example.org