U.S. Marines have located a complex of tunnels underneath an Iraqi nuclear complex – apparently missed by U.N. weapons inspectors – discovering a vast array of warehouses and bombproof offices that could contain the "smoking gun" sought by intelligence agencies, reported the Pittsburgh Tribune-Review.

The Iraqi Atomic Energy Commission's Al-Tuwaitha facility is located 18 miles south of Baghdad.

Fox News Channel is reporting that the tunnels may contain weapons-grade plutonium.

"I've never seen anything like it, ever," said Marine Capt. John Seegar. "How did the world miss all of this? Why couldn't they see what was happening here?"

Marine nuclear and intelligence experts say that at least 14 buildings at Al-Tuwaitha indicate high levels of radiation and some show lethal amounts of nuclear residue, according to the Pittsburgh daily. The site was examined numerous times by U.N. weapons inspectors, who found no evidence of weapons of mass destruction.

Marine combat engineers guard Iraqi Atomic Energy Department (Carl Prine/Pittsburgh Tribune-Review)

"They went through that site multiple times, but did they go underground? I never heard anything about that," said physicist David Albright, a former International Atomic Energy Agency inspector in Iraq from 1992 to 1997.

In a 1999 report, Albright said, "Iraq developed procedures to limit access to these buildings by IAEA inspectors who had a right to inspect the fuel fabrication facility."

"On days when the inspectors were scheduled to visit, only the fuel fabrication rooms were open to them," he said in the report, written with Khidhir Hamza, an Iraqi nuclear engineer who defected in 1994. "Usually, employees were told to take to their rooms so that the inspectors did not see an unusually large number of people."

Chief Warrant Officer Darrin Flick, the battalion's nuclear, biological and chemical warfare specialist, said radiation levels were particularly high at a place near the complex where local residents say the "missile water" is stored in mammoth caverns.

"It's amazing," Flick said. "I went to the off-site storage buildings, and the rad detector went off the charts. Then I opened the steel door, and there were all these drums, many, many drums, of highly radioactive material."

Noting that the ground in the area is muddy and composed of clay, Hamza was surprised to learn of the Marines' discovery, the Tribune-Review said. He wondered if the Iraqis went to the colossal expense of pumping enough water to build the subterranean complex because no reasonable inspector would think anything might be built underground there.

"Nobody would expect it," Hamza said. "Nobody would think twice about going back there."

Michael Levi of the Federation of American Scientists said the Iraqis continued rebuilding the Al-Tuwaitha facility after weapons inspections ended in 1998.

"I do not believe the latest round of inspections included anything underground, so anything you find underground would be very suspicious," said Levi. "It sounds absolutely amazing."

The Pittsburgh paper said nuclear scientists, engineers and technicians, housed in a plush neighborhood near the campus, have fled, along with Baathist party loyalists.

"It's going to take some very smart people a very long time to sift through everything here," said Flick. "All this machinery. All this technology. They could do a lot of very bad things with all of this."

Seegar said his unit will continue to hold the nuclear site until international authorities can take over. Last night, they monitored gun and artillery battles by U.S. Marines against Iraqi Republican Guards and Fedayeen terrorists.

The offices underground are replete with videos and pictures that indicate the complex was built largely over the last four years, the Tribune-Review said.

Pentagon spokeswoman Torie Clark told reporters she has no specific information about the discovery.

"I'm only aware of the report from the embedded reporter," she said.

Clark was asked about the timeline of testing the material found.

"Every situation is different," she said. "You often have different circumstances. Sometimes things test positive and then it turns out to be negative. We're taking our time and we remain focused on the primary task of winning the war. It doesn't mean we can't do other things; we do. But we will take our time and do it properly."

Iraq began to develop its nuclear program at Al-Tuwaitha in the 1970s, according to the Institute for Science and International Security. Israel destroyed a French-built reactor there in 1981, called "Osiraq," and a reactor built by the Russians was destroyed during the 1991 Gulf War.

In his 2000 book "Saddam's Bombmaker," Hamza revealed Saddam's secret plans for the nuclear complex at Al-Tuwaitha:

From my office window in the Nuclear Research Center, I could see just a slice of what Saddam's oil money had built in less than a decade: a sprawling complex of nuclear facilities, scattered over ten square miles, poised to deliver us the bomb. It was called al-Tuwaitha, in Arabic "the truncheon."

… Below my floor was fifty thousand square feet of office space and laboratories, sparkling with new equipment, where hundreds of technicians were running nuclear experiments. Outside to my left was our chemical reprocessing plant, where we would enrich fuel for a plutonium bomb. Down the street was our domed Russian reactor, newly renovated with Belgian electronic controls, which made it capable of generating radioactive material for nuclear triggers. Past that was our French-supplied neutron generator, and next to that our electronics labs, and then a four-story building that handled spent nuclear fuel, full of hot cells and new remote-controlled equipment overseen by platoons of white-jacketed technicians. All this was a long, long way from the dining room table where we'd scratched out our first memo for a bomb in 1972.

Rising up behind my office, however, was al-Tuwaitha's jewel in the crown, the aluminum dome of the French reactor, glittering in the blue desert sky. Osiraq was the most advanced reactor of its kind, crammed with such up-to-date equipment and technology that visitors were amazed that the French had ever agreed to sell it to us. Little did they know that the acquisition of Osiraq, an incredible feat on its own, was merely a decoy: Saddam wanted us to copy the French design and build another, secret reactor, where we would produce the bomb-grade plutonium beyond the prying eyes of foreign spies and inspectors – the same thing to him.

But it was not to be. On June 7, 1981, Israel sent eight F-16 warplanes almost 700 miles over Jordanian, Saudi and Iraqi air space for hours without detection. By flying in tight formation, they generated a radar signal resembling that of a commercial airliner. Upon identifying the Osiraq nuclear plant, and catching Iraqi defenses by surprise, the Israeli pilots managed to demolish the reactor in one minute and 20 seconds.

At the time, Israel's audacious preemptive strike was almost universally condemned, but later praised by many for helping thwart Iraq's development of nuclear weapons.

Despite this and other setbacks, says Hamza, Saddam persisted in his quest for a nuclear bomb. In testimony before Congress last August, Hamza – the architect of Iraq's atom bomb program – said that if left unchecked, Iraq could have had nuclear weapons by 2005.