SAN DIEGO, CALIF.

U.S. Border Patrol agents, sweating under a hot Texas sun, squared off against an array of fences. They swung axes at posts, used blowtorches to melt steel, tore through sheet metal with crowbars and scaled walls with ladders.

Government engineers with the agents rammed remote-control SUVs loaded with 10,000 pounds of sand into the barricades at 40 miles per hour.

Together, in a nine-week project called Fence Lab, they were trying to solve one of the nation's most vexing problems -- how to find fencing strong enough to protect the U.S. from one of the largest human migrations in history but sensitive enough to the fact that Mexico and the United States are friendly nations.

Consider the government's requirements: The fence must be formidable but not lethal, visually imposing but not ugly, durable but environmentally friendly and economically built but not flimsy.

"It's not that simple," said Collin Sloan, whose Idaho company was among those submitting designs to Fence Lab.

Sloan has studied guard towers, machine guns and razor wire at border defenses around the world.

"Other countries are a lot more into intimidation," he said. "This is the only humane border fence being constructed."

The largest fence expansion in the history of the United States' southwest border is under way, with more than 70 miles erected in 2007 and 225 miles planned for 2008.

Often lost amid the debate over how to control the border are the physical barriers themselves. Different terrains call for different approaches and, over the years, a patchwork of different fence styles has been built along the border.

In the shadow of the Huachuca Mountains in Naco, Ariz., a double-layer steel mesh barricade stretches across the frontier like a sheet of honeycomb netting.

Alongside small towns in California and Arizona, tall steel tubes form what look like giant picket fences.

Outside Yuma, Ariz., a wall of steel plates burns as hot as a skillet in the desert heat.

Fence design is constantly evolving. The mesas and canyons along the San Diego-Tijuana, Mexico, border form a museum of successes and failures.

In the early 1990s, agents lined 14 miles of frontier with ¼-inch-thick carbon steel panels originally used as aircraft landing mats during the Vietnam War. They formed a vertical wall with ridged sides exposed, in effect creating a ladder for immigrants to climb.

When agents added angled metal screens onto the plates to make climbing more difficult, migrants hooked rebar ladders to the tops and began streaming over.

None of the new barriers has angled parts or horizontal slats. The newer fencing is transparent because border agents complained that solid steel mats blocked their view of smugglers staging crossings in Mexico.

In 2007, the Department of Homeland Security began the search for new fences, inviting private contractors to submit ideas.