As U.S. military troops push toward Baghdad with the most modern equipment, technology and training money can buy, the Iraqi leadership will rely on a defense that is ancient as warfare itself: underground tunnels and bunkers.

Over the past 20 years, Iraqi President Saddam Hussein is believed to have constructed an elaborate series of underground tunnels and bunkers around Baghdad where he, his leadership and the elite troops that guard them are able to move about virtually undetected and nearly impervious to U.S. munitions, according to Iraq experts, a former Iraqi scientist and Western construction officials whose companies helped build the warrens.

Among the more extensive tunnel complexes are those described by Hussein Shahristani, once Iraq's top nuclear scientist, who was tortured and imprisoned in Iraq. Shahristani, who escaped Iraq during the first Gulf War, said that subway plans developed by several foreign firms were actually used by the Iraqi military to hide and transport the country's chemical and biological weapons. Shahristani told CBS's "60 Minutes" in February that he believed Iraq had "more than 100 kilometers of very complex network, multi-layer tunnels."

Other Iraqi exiles and former Western construction company officials have said all of Saddam's presidential palaces include deep, hardened bunkers, some of them massive in scale. Included among them is one $70 million palace-and-bunker complex that is protected by thick layers of concrete, steel and blast doors that Wolfgang Wendler, a German engineer who helped build it, described as being able to withstand a Hiroshima-size explosion.

By Thursday, U.S. military officials reported that U.S. and British aircraft had bombed Hussein's known presidential compounds and residences, often with "bunker-buster" bombs that burrow deep underground before exploding.

Many of Iraq's military tunnels are believed to have been built by Aeroinzenjering, a Serbian engineering firm once run by the military of the former Yugoslavia. Hussein maintained a close relationship with Communist leader Tito (Josip Broz), and with Slobodan Milosevic, whose underground tunnels and bunkers bedeviled U.S. and NATO commanders during the 1999 Kosovo air war.

The company's Web site boasts that Aeroinzenjering has constructed a 450,000-square-meter facility described as "special airport underground fortification structures, sheltering complete military units and equipment." But it does not say where the facility was built.

Iraq, North Korea, Syria, Yugoslavia and the al Qaeda network in Afghanistan have all succeeded in hiding their leaders -- including Osama bin Laden -- as well as significant military units and weapons production facilities in deep, reinforced, cement-lined bunkers and tunnels that are as hard to find as they are to destroy.

Finding tunnels and bunkers has become a priority for U.S. military and intelligence in the past several years.

"It is an activity that takes more of our time as nations and potential adversaries move from above ground to below," said David Burpee, spokesman for the National Imagery and Mapping Agency, which has spent decades mapping the world's surface in great detail, and is now trying to do the same to the Earth's underground.

A batch of technological innovations is allowing the U.S. government to better understand what's below the surface, Burpee said.

Among them are seismic devices akin to big hammers that pound the ground and bounce back a signature like radar.

Experts also use gravimeters, which measure the variations in the gravitational field between two or more points to help pinpoint underground installations. Special high-flying spy cameras that measure thermal energy and chemical releases are used to find clues that may indicate the presence of underground activity.

The U.S. military is also developing new targeting techniques, fighting tactics and intelligence gathering about underground facilities.

In 2002, during the war in Afghanistan, the Navy established the Tunnel Warfare Center in California's Mojave Desert where "troops are trained in finding, locating and perhaps destroying" tunnels, said spokeswoman Doris Lance. The 1.1 million-acre test and training range has 300 miles of abandoned mines that resemble the bunkers, vertical shafts and multilevel tunnel complexes of Afghanistan and Iraq, she said.

In 1965, the United States pioneered the use of hardened tunnels in the nuclear age, when with Canada it opened the operations center for the nuclear attack-resistant North American Air Defense Command (NORAD) carved deep into Cheyenne Mountain.

Research director Margot Williams contributed to this report.