The border barrier dips and curves, zigs and zags, hugging the mountain's contours like a slimmed-down version of the Great Wall of China.
Among the costliest stretch of fencing ever built on the U.S.-Mexico border, the 3.6-mile wall of steel completed last fall is meant to block trafficking routes over Otay Mountain, just east of San Diego.
People seeking to enter the country illegally have hiked the scrub-covered, tarantula-infested peak for years, trying to get to roads leading to San Diego.
"We're no longer conceding this area to smugglers," said Jerome C. Conlin, a U.S. Border Patrol spokesman.
But critics are bewildered. Why, they ask, would people determined to scale a rugged, 3,500-foot peak be deterred by an 18-foot-high fence? They also point out that the Department of Homeland Security deemed it unnecessary in 2006.
"I think it's a Bush-era boondoggle that will have almost no consequence in terms of stemming the flow of immigration," said Char Miller, director of the environmental analysis program at Pomona College. "It was a political decision that took in no account of the environment itself, and in the process damages what was once a pretty remarkable landscape."
The $57.7-million project is one segment in the massive expansion of border infrastructure approved by Congress during George W. Bush's presidency. Homeland Security has erected fencing in small towns, remote valleys and high-desert mesas from the Pacific Ocean to Texas.
At about $16 million a mile, the Otay Mountain barrier cost about four times as much as similar border fencing built during this expansion, according to the U.S. Government Accountability Office.
The Border Patrol's San Diego sector was already one of the country's most heavily fortified frontiers before the mountain fence was constructed, with about 40 of the sector's 60 miles lined with vehicle or pedestrian barriers.
The fencing shifted immigrant flows to remote areas in the backcountry east of San Diego. But some migrants decided to climb Otay Mountain because of its proximity to a warehouse district in San Diego and its easy access on the Mexican side, where the Tijuana-Tecate toll road lies only a few hundred yards away.
Immigrants dropped off at staging grounds off the toll road headed up steep trails into the U.S. Their hikes through canyons and over the arid peak could take up to three days. With limited road access on the mountain, agents simply waited for people to descend to make arrests.
The lack of fencing did not seem to be a problem, said then-U.S. Border Patrol spokesman Richard Kite, interviewed in a 2006 article in the Arizona Daily Star. At Otay Mountain, "you simply don't need a fence. It's such harsh terrain it's difficult to walk, let alone drive," Kite said. "There's no reason to disrupt the land when the land itself is a physical barrier."
The agency said it changed course after reevaluating conditions in the area. Daryl Reed, a current Border Patrol spokesman, said strategies are in constant flux depending on quickly shifting migrant flows and smuggler activity.
"As we continue in our mission, we're always reevaluating situations," Reed said. "We're always going to adapt and change."
One analyst suggested that pressure from Congress to complete about 700 miles of fence led federal officials to approve some questionable projects.
"There's no question that there's tactical justification for certain fencing, but when you set up a target like that, it inevitably means that they're going to build fencing where the tactical justification is weak, and this sounds like one of those places," said Edward Alden, a senior fellow at the Council on Foreign Relations.
But others doubted that border authorities would spend resources in an area that didn't need it.
"If there were other better places to build fencing, then I'm confident the Border Patrol would build it there," said Mark Krikorian, executive director of the Center for Immigration Studies.
When the federal government broke ground last year, environmental groups, including the Sierra Club, said the project would damage the Otay Mountain Wilderness. Portions of the fence and the 5-mile access road lie in the federally protected area.
The federal government, trying to expedite construction of border fencing, waived more than 30 environmental laws in 2008, including the Wilderness Act, Clean Water Act, Endangered Species Act and others that environmentalists said applied to the Otay area.
Contractors had to cut roads, remove boulders, bulldoze hillsides and remove about 530,000 cubic yards of rock to build the Otay fence, which consists of steel posts 4 inches apart topped with metal plates.
It's not clear whether the fence has been a deterrent.
Since the barrier's completion in October, illegal activity has declined and there have been few signs of people trying to cut or breach the fence, authorities say.
"Having this fence here is definitely going to slow them down. . . . It increases our probability of catching them," said Conlin, the Border Patrol spokesman.
But others say the fence's effectiveness hasn't been truly tested because fewer immigrants have been attempting to cross anywhere on the border due to the economic slowdown.
The funding, they said, could have been better spent hiring more agents or building infrastructure in other areas.
When the economy improves, the mountain will once again draw immigrants, fence or no fence, said Pedro Rios, director of the American Friends Service Committee in San Diego.
"It seems to me, if someone is able to climb the mountains in the Otay Wilderness, a 15-foot wall will not make a difference," he said.