Holding the low ground: daunting challenges face those waging subterranean warfare.

To take a desktop trip into a clandestine tunnel, one only needs to visit the YouTube.com Web site and punch in the words "Gaza" and "tunnel."

In the clip, Israeli Defense Forces guide the viewers into a deep shaft that opens up into a passageway reinforced with wooden beams. It leads into Egyptian territory, and is one of many used to smuggle weapons, people and other contraband.

The IDF also had to deal with tunnels and deep bunkers in Southern Lebanon last summer when it squared off against Hezbollah fighters launching rockets into their territory. Reports said the underground complexes took years to construct.

But Israel isn't the only nation dealing with underground passageways or facilities.

In South America, the Colombian military found a network of tunnels used by Revolutionary Armed Forces of Colombia rebels, better known as FARC.

The Department of Homeland Security has unearthed several tunnels under the Mexican and Canadian borders. The largest, more than one mile long, was discovered last year near Otay Mesa, Calif.

Potential U.S. adversaries Iran and North Korea have reportedly built underground facilities to hide their alleged weapons programs. In the case of Iran, what kinds of weapons are being developed, and how extensive these facilities are, are subjects of much debate in the intelligence communities, but the advantages of keeping military assets out of the reach of air strikes or spy satellites, is unquestioned.

Coalition forces have had to deal with caves and tunnels in the mountains of Afghanistan as they chase al-Qaida and Taliban forces. If Osama bin Laden is hiding in Pakistan's tribal territories, as many believe, he's had five and a half years to dig himself a well-fortified and comfortable hole.

All this has not gone unnoticed by the U.S. defense community. Gen. John Abizaid, the former commander of U.S Central Command, indicated before a group of Washington-based reporters last fall that he has given the subject some thought. Overall, he was disappointed with the tools the military has to detect underground passageways.

"On a scale of one to 10, the technology is a four. We need more ability to see better underground," he said.

The public may associate clandestine tunnels with jailbreaks, but their use in warfare, both for offense and defense, is not new.

In 1864, Union troops dug a 500-foot tunnel under the Confederate line at Petersburg, Va. They planted 8,000 pounds of gunpowder beneath the fortifications and blew out an enormous crater. Too enormous, it turned out. Union soldiers could not fight their way out of the hole after attacking.

During World War II, the Japanese used a 16-mile network of tunnels to defend Iwo Jima. They ultimately lost the battle, but their fortifications made naval bombardment ineffective and they inflicted staggering casualties on the Marine Corps.

The Viet Cong's extensive use of tunnels for offensive purposes in southern Vietnam is a more recent example of how underground operations have been incorporated in asymmetric war tactics.

In this low-tech era, the United States sent in a small cadre of so-called "tunnel rats," lithe soldiers who bravely went down into the unknown with little more than a handgun, bayonet and flashlight, according to in Tom Mangold and John Penycate's book, "The Tunnels of Cu Chi."

The expertly engineered tunnels, employing beveled trap doors and cleverly disguised entrances, made them difficult to defeat by flooding, gasses or flamethrowers.

Fortunately for the VC and North Vietnamese regulars, the soil laced with laterite clay was as hard as cement, making it nearly impervious to aerial bombardment. The enemy was able to construct mess halls, field hospitals and tactical operation centers on four separate levels only a few meters beneath the earth.

The Cu Chi tunnels are tourist attractions nowadays. And while some may believe them to be relics of the past, descriptions in the media of the FARC complex in Colombia and the Hezbollah tunnels in Southern Lebanon are strikingly similar to those in Vietnam 40 years ago.

The Dominican Today Web site quoted a Colombian military officer who described the underground camp as "a sophisticated architectural work." It had rooms, tunnels and escape exits. FARC is responsible for 60 percent of the cocaine smuggled into the United States, according to the Justice Department.

The Hezbollah tunnels in Southern Lebanon had ceilings high enough to walk under, ventilation, lighting systems and bathrooms, an IDF commander told Israel's Yedioth Ahronoth newspaper. "Dozens of command bunkers were built inside the network, divided into two or three rooms each," the commander said.

The Cu Chi tunnels played a vital role in launching the Tet Offensive, which is seen by many historians as a military defeat for the North Vietnamese, but a turning point that eventually led to the United States withdrawing its troops. Hezbollah used tactics similar to the Viet Cong. They launched their hit-and-run attacks--in this case rockets aimed at Israel--then quickly retreated underground.

Today, the technology to detect and defeat underground facilities has improved, but is far from perfect. Research has been going on for decades, but as one scientist presenting a paper at an American Geophysical Union tunnel detection seminar last year wrote, "despite heavy funding from the military since the early 70s, geophysicists have not produced tools that are simple and practical enough to meet military needs."

Until recently, the defense community has had disparate subterranean warfare programs. The Army Corps of Engineers' engineer research and development center in Vicksburg, Miss., has studied detection technology for several decades. The Air Force has taken the lead on bunker buster munitions. The Defense Advanced Research Projects Agency is currently running three programs in the areas of sensors, hardened facility defeat and cross border tunnels. The agency is developing wall-penetrating radar sensors "and other techniques for detecting and mapping underground structures," its deputy director, Robert Leheny wrote in Military Technology magazine.

The Defense Department will have some help. The drug smuggling tunnels found under the U.S. land border have prompted the Department of Homeland Security to jump into the business. Undersecretary of Science and Technology Jay Cohen said in a recent speech that tunnel detection will be a research and development priority this year.

DHS and the military have stated their intention to cooperate under Joint Task Force North's futures group. It has sponsored at least two conferences on the topic.

Once a tunnel or cave is detected, robots are an alternative to the "tunnel rats" of the Vietnam War era. They have been sent into caves in Afghanistan to check for insurgents, booby traps and weapons caches. While no robot can yet fully mimic the movements of a human body, the short guys in today's combat units should be grateful. In Vietnam, they were the ones ordered into the unknown.

Munitions that can penetrate hardened facilities, like the GBU-28 bunker buster, have been around for decades. Forcing a bomb to cut through the earth, delaying its detonation until a specified depth, is a tricky engineering problem, though. For larger and deeper complexes that may harbor facilities where weapons of mass destruction are manufactured, there are tactical nuclear systems such as the B61-11. Military and political leaders would, of course, have to wrestle with the taboo of using such weapons.

In urban areas, where employing bunker busters is not an option, destroying smaller tunnels and caves used by groups such as insurgents, is complicated. Flamethrowers, used extensively at Iwo Jima, are also a relic of the past. They were effective in World War II in rooting out German and Japanese soldiers from bunkers, pillboxes and caves, but the Defense Department stopped requisitioning them in 1978, mostly out of humanitarian concerns. The M202A1 incendiary rocket launcher replaced it, but is not specifically designed for tunnels.

Before measures can be taken to search or destroy an underground structure, it must first be located. And that has proven to be the most challenging problem.

Because of the many types of soil and geological formations found in the world, it's doubtful there will ever be a silver bullet technology capable of detecting subterranean structures, experts have said.

The general location of the Otay Mesa drug smuggling tunnel, for example, came from a tipster, not technology, an Immigration and Customs Enforcement agent told National Defense last year. Representatives of Joint Task Force-North, under U.S. Northern Command, brought in sensors to pinpoint its exact location.

Detecting a tunnel in urban areas, with pipes and sewer systems that can create false positive readings, poses its own set of problems. Acoustic sensors will pick up a city's background noise. Seismic sensors may indicate ancient streambeds instead of man-made structures.

Aerial or space-based sensors may be able to find entrances. A tunnel or cave opening may emit temperature fluctuations that can be detected. Disturbed soil, which could be picked up by fluctuating colors from a hyper-spectral sensor, could also indicate the presence of someone excavating a tunnel. Neither of these methods would indicate depth or direction of a tunnel, though. Equally tricky, analysts have pointed out, is battlefield damage assessment. Once a bunker buster penetrates an underground complex, how can commanders tell if they hit the target?

To detect tunnels, the defense community must enter the unfamiliar world of geophysics. During the American Geophysical Union seminar last year, scientists presented proposals on esoteric subjects such as soil databases, electromagnetics, electrical resistivity imaging and shear-wave velocity.

The Army Research Office hosted a similar seminar in Vicksburg, Miss, last year to address what it calls the "severe technical challenge" of creating easily-deployable, practical and reliable tunnel detection technology. Part of the agenda addressed the need to make the research community aware of the Defense Department's acute need to peer underground.

The Air Force is addressing the problem, but not all its ideas involve ground-penetrating warheads. The use of underground passageways by insurgents is of interest to the service, as a recent request for information from its research lab indicated. There is a need for a device that can burrow underneath places like hospitals or residential areas where bombs cannot be used, it said.

"A subterranean vehicle could engage these types of targets in an effective manner ... It could be deployed at a safe distance from the target and autonomously navigate itself to the target while detecting, identifying, and then avoiding buried obstacles such as pipes, wires, boulders and even other buildings," the document states.

So someday, along with UAVs, (unmanned aerial vehicles), and UUVs (unmanned underwater vehicles), the Defense Department may add USVs (unmanned subterranean vehicles) to the list of acronyms that comprise the Pentagon's robot army.