In late May 1991, a few months after Iraq's loss in the Persian Gulf War, four Iraqis drove up to a U.S. Marine checkpoint in the Kurdish north of Iraq, near the town of Dohuk. One occupant claimed to be Saddam Hussein's top nuclear scientist. The U.S. whisked him away to Turkey and then Munich for debriefing, during which he claimed that the coalition bombing campaign had missed a number of key facilities, including a large underground uranium enrichment facility inside a mountain north of Mosul.
Like many defectors, he was not exactly telling the truth. He was not Saddam's chief nuclear scientist, but a physicist who had worked at the Ash Sharqat facility, home to a calutron, a World War II-era device for enriching uranium. There was no sophisticated, secret underground uranium enrichment facility.
Not all reports of underground facilities are the figment of someone's imagination, which is why in 1997, then-Director of Central Intelligence George Tenet ordered the Defense Intelligence Agency to establish the Underground Facilities Analysis Center (UFAC). There, the motto is "unearthing the truth in defense of our nation." Its logo shows a sword crossed with a pick laid on top of an image of the Earth. To give the analysts at UFAC more to go on, the U.S. is working on technologies to study underground facilities in greater detail from the air and space and with seismic sensors.
A May 2000 Air War College paper raised the possibility of equipping a UAV with a gradiometer, a GPS receiver and a means of transmitting the gradiometer data to an airborne or space platform. Gradiometers measure the slight changes in the Earth's gravitational tug when dirt, rock, ice or water are moved around. The gravity maps, or gradients, they produce have been employed by the U.S. Navy's submarine force to stealthily detect underwater obstacles.
If a country were to dig tunnels through a mountain the gravity gradient would reveal the presence of cavities. The paper noted that the "ability to program a UAV to autonomously accomplish such a mission, from take off to landing, is feasible and in fact serves as the fundamental concept for the Air Forces' Global Hawk UAV."
The U.S. Defense Advanced Research Projects Agency must have been reading carefully. In June 2006, DARPA awarded BBN Technologies of Cambridge, Mass., a $3.13 million contract to help develop the Low Altitude Airborne Sensor System to be tested on low-altitude UAVs and manned aircraft. The system will employ passive electromagnetic, acoustic and gravity gradiometer sensing.
DARPA is also funding the Airborne Tomography using Active Electromagnetics (ATAEM) program. The program focuses on developing an active electromagnetic system for airborne imaging of subsurface structures. Inspired by the geophysical exploration industry, an ATAEM system illuminates the ground with electromagnetic energy and interprets the resulting distortions of the electric and magnetic fields to detect and characterize hidden underground structures.
The work has a very real-world motivation. In 1996, the U.S. alleged that Libya was building an underground chemical weapons facility in an excavated mountain near Tarhunah, 40 miles south of Tripoli. A year later, information surfaced about Russian efforts to build what might have been a nuclear command and control center within Yamantau Mountain in the Urals, about 850 miles east. In 1997, Russia was working on a relocation bunker outside of Moscow, a facility hundreds of feet underground designed to permit the nation's rulers to survive a nuclear attack.
More recently, Britain's Daily Telegraph reported on a network of underground tunnels at China's Sanya naval base on the southern tip of Hainan Island. The tunnels, the paper reported, were estimated to be 60 feet high and built into hillsides around the base, leading the paper's sources to fear they could lead to caverns capable of concealing up to 20 nuclear submarines from U.S. and other spy satellites.
In an August 2007 presentation to DARPA's Systems and Technology Symposium, Joseph Durek, the chief of DARPA's Strategic Technology Office, told his audience: "It's estimated that there are thousands of strategic underground facilities spread across the dozens of countries, with a significant fraction of them hiding weapons of mass destruction, missiles or military leadership." Or, as Maj. Mark Easterbrook wrote in a 2005 article in Pathfinder, the in-house magazine of the National Geospatial-Intelligence Agency, "The world has gone underground." A paper written for the Air War College characterized deeply buried underground targets as one of the most difficult operational challenges to confront U.S. military forces in the 21st century.
While there has been mystery about some underground sites, there is no mystery about why nations have built or tried to build such sites. In the extreme, and with some luck, an underground facility could be built and operated without the U.S. being aware of its existence. Without human intelligence or communications intercepts to make America suspicious, U.S. imagery satellites might pass overhead regularly, unaware of what was going on underground. A country also could employ concealment techniques to include panchromatic deception (the use of subdued paint schemes, rather than color) for surface items, three-dimensional decoys to conceal the location of external support equipment for the underground facility, and thermal imaging deception to prevent identification of a facility from the heat generated by air vents, water pipes or electrical conduits. Thermal imaging deception techniques include heavy vegetation and thermal blankets as well mixing cooler air with warmer exhaust air.
But even if the U.S. could not be kept completely in the dark, knowledge about the activities at a suspicious site could be limited by ensuring that whatever goes into or out of the facility is transported underground.
There is also another reason a nation might go underground: immunity from aerial attack, including nuclear attack. While President Bush's Defense Department has failed to persuade Congress to fund an earth-penetrating nuclear weapon called the Robust Nuclear Earth Penetrator, the U.S. arsenal does include the BLU-109B, a 2,000-pound bomb designed to defeat an enemy's most critical and hardened targets, according to the Air Force. An Air Force fact sheet says the weapon can penetrate up to 6 feet of reinforced concrete to reach the deep interior of hardened sites, where a delayed action fuse can detonate 550 pounds of high- explosive Tritonal to ensure destruction of the target.
As much as foreign nations want to conceal the existence of underground sites, or hide the details of activities there, the U.S. wants to penetrate the veil of secrecy. The constellation of radar and electro-optical imagery satellites operated by the National Reconnaissance Office is responsible for some of what U.S. intelligence analysts know about such facilities. Currently that constellation consists of at least three Enhanced Crystal System satellites, which, according to a retired official have both electro-optical and infrared imagery capabilities, and two radar imagery satellites. Crystal is a code name coined in 1982 for the KH-11 imaging satellites.
The drawing of Libya's Tarhunah plant that appeared in a 1996 Defense Department publication was based on satellite imagery. U.S. reconnaissance satellites also provided images of the construction at Yamantau mountain and other Russian underground bunkers. Imagery in early 1997 showed digging at the Yamantau underground complex as well as at each of the site's above-ground support areas.
But as the author of the Pathfinder article noted, "Detection and characterization are difficult at best. Determining function and construction features presents complex collection, analysis and engineering challenges." The case of an underground site in North Korea certainly illustrates that point. Possibly by the end of 1997, and certainly by the summer of 1998, U.S. imagery satellites detected activity at a site known as Kumchang-ri, 25 miles northeast of Yongbyon. The imagery showed a massive tunneling complex, involving about 15,000 workers swarming around the site, digging into the mountainside. The immediate fear among U.S. officials was that North Korea was building either a nuclear reactor or reprocessing plant under the mountain. But the high-resolution imagery produced by American spy satellites could not provide any further clues. After prolonged negotiations, including a U.S. threat to terminate the 1994 Agreed Framework, North Korea gave a 14-person American inspection team permission to look under the mountain. The team began its visit May 20, 1999, concluding on the evening of May 22. When the team left, it knew little more than when it arrived: There was an extensive, but empty, underground tunnel complex for the members to see. There were no workers, no construction activity and no equipment.
Of course, the U.S. intelligence community has for years been well aware of the challenges presented by underground facilities. On the first two days of December 1993, the CIA's Office of Scientific and Weapons Research hosted an Underground Military Facilities Symposium in the agency's auditorium to study how the U.S. might reduce uncertainty about the characteristics and purposes of those facilities.
But establishment of the UFAC has been critical. The center is staffed by representatives of the DIA's measurement intelligence and technical collection directorates; the Defense Threat Reduction Agency; the National Security Agency, and the National Geospatial-Intelligence Agency. The center also works with the U.S. Geological Survey, the U.S. Strategic Command joint intelligence center, the CIA, the National Reconnaissance Office and the Energy Department laboratories.
The center's key function, as spelled out in its charter, is to "establish and sustain a coordinated collaborative understanding of the essential features of adversarial [including terrorist] underground facilities through the world." Its staff maintains folders on each facility filled with information that could be used in intelligence assessments as well as targeting packages. Guidance for the center is provided by the Underground Facility Senior Steering Group, co-chaired by Lawrence K. Gershwin, national intelligence officer for science and technology, and a representative of the undersecretary of defense for intelligence.
Sources of intelligence for the center's analysts span the gamut from human and communications intelligence (including defectors) to satellites.
Overhead assets can be used in a variety of ways to gather intelligence about underground facilities. Traditional imagery sensors can spot excavation of sites such as Tarhunah. The National Geospatial-Intelligence Agency also uses that imagery to produce high-resolution terrain information that assists the center in identifying and evaluating underground facilities. Minor changes in elevation at a specific location might indicate underground construction. In 1998, to help refine the technique, the National Imagery and Mapping Agency, now known as the NGA, began construction of the HRTI Test Range at the Nevada Test Site.
Multispectral or hyperspectral imaging sensors carried on commercial or intelligence satellites can detect the traces left behind by humans and machines from construction efforts, attempts at camouflage and the presence of exhaust gases. Infrared imagery sensors, such as those carried by the Enhanced Crystal System satellites, can detect the vent duct arrays or the heat generated by underground facilities that are sufficiently close to the surface. The blue-band filter on a nonclassified environmental satellite such as Landsat can also detect the smoke and gases that are emitted from underground vents.
Ground sensors are an alternative to overhead reconnaissance. The Air Force Technical Applications Center maintains seismic detection facilities around the world to monitor nuclear detonations. These might provide intelligence about underground construction if sufficiently large signals were generated in the clearing of space for construction of underground installations.
Small ground sensors, possibly disguised as rocks or other natural objects, could be covertly deployed by UAVs or aircraft during nighttime missions. In a 1999 report by a Defense Department advisory group, "Characterization of Underground Facilities," the authors noted that placing sensors was becoming more attractive due to miniaturization of electronics, sensors and communications technology, and other improvements. Another BBN project has involved the development of a prototype of a Counter Underground Facilities Unattended Ground System. Documents describe it as consisting of "multiple, networked ground nodes of electromagnetic, acoustic and seismic sensors" for underground monitoring and target characterization.
These and other collection efforts are guided by theoretical work on the physical signatures associated with building underground facilities and operating them. In early June 2004, Wendee Brunish, a scientist from Los Alamos National Laboratory, visited the Virginia headquarters of the NRO to give a presentation on a new technique for building 3-D models of underground facilities. The technique was part of the lab's effort to develop a geological assessment methodology for underground targets. The technique reportedly allows differentiation of structural blocks, stratigraphic layers, as well as facility infrastructure. According to a lab news release, Brunish showed results for an unspecified "underground target of great interest in a foreign country."
How well the staff of the Underground Facilities Analysis Center and other analysts are doing at identifying and characterizing underground facilities is difficult for any outside observer to determine. It might even be difficult for those with full access to know. The U.S. cannot, by definition, know how many facilities it has failed to identify. But it would appear that the search for underground facilities is far from over. The author of the Pathfinder article estimated that 10,000 underground facilities had been identified and a suspected equal number were yet to be discovered. å
Jeffrey T. Richelson is a senior fellow with the George Washington University National Security Archive. He is the author of several books, including "The Wizards of Langley: Inside the CIA's Directorate of Science and Technology."