TEL AVIV—The methods the Israeli Defense Forces (IDF) used to discover the tunnels that connected Gaza to Israel have been shrouded in secrecy. An IDF spokesman said that the army had recently examined more than 100 different technological solutions, even though it is not the first time Israel has faced such a challenge.

Even back in 2004, the army was forced to respond to the smuggling of weapons through the tunnels under the Rafah crossing between Gaza and the Egyptian border; and it seemed as if every geologist in the country had been summoned for the task.

Israel is not alone when it comes to problems with tunnels. Over the past 20 years, the United States has also been forced to face the more than 200 tunnels that have been discovered along the U.S.-Mexican border. Some are up to 35 meters deep and many kilometers long, used for smuggling weapons, drugs and humans. A one-kilometer-long tunnel that stretched between Arizona and Mexico was found to have lights, was large enough for several people to walk side-by-side and even had air-conditioning, rails for heavy merchandise, telephone lines, and reinforced walls.

In 2006, the U.S. Department of Homeland Security launched a long-term project to develop technology to prevent smuggling. Still, the project's chief Edward Turner says the tunnels that have been discovered were typically thanks to human intelligence, rather than any particular technology. “There is pressure coming from the government to supply some kind of magical device that can be taken to the border and discover a tunnel in ten minutes,” he said. “It just cannot happen like that.”

John Verrico, responsible for new technologies for DHS, explained that a sensor that works for one kind of land, or one kind of weather, would not work for another. “Everything can have an influence: the season of the year, land pollution, air bubbles or variations in the geological layers,” he explained. “If you do not understand the land you are testing, you will have a big problem differentiating between the natural features and the man-made tunnels.”

Among the existing tools for unearthing tunnels:

Ground-Penetrating Radar
One of the first solutions that scientists and army officials turned to was radar, a long reliable technology that is accessible, accurate and operates from above-ground. In order to perfect the results, a ground-penetrating radar uses very strong electro-magnetic pulses of radio waves in order to map the land, and creates a three dimensional picture of all the layers and rocks that are below the surface. This type of radar is currently used by the United States, Israel and even Egypt around the border with Gaza and the Rafah crossing, but still has many defects.

The most up-to-date radar, which weighs 12 kilograms and costs several thousand dollars, is able to identify empty spaces within close range of the surface. It is, however, unable to find anything below 10 meters of depth.

Microphones
Another solution to locate tunnels based on seismographic principles. Instead of trying to map the ground, this solution listens to noises and vibrations caused by the digging of new tunnels or by the transporting of goods and people through them. These sound waves create vibrations in the ground that seismographic equipment, such as special microphones, can detect. This technology is being developed by the IDF as an experimental project that costs $60 million. It is still not considered very accurate, and tends to send false alarms.

Infrared
The main focus of the U.S. administration today is the development of a technology based on infrared radiation that locates anomalies in the heat dissipation on the surface, indicating the existence of tunnels. In other words, the supposition is that there will be a difference in temperatures where there is a tunnel. Furthermore, infrared sensors have become quite cheap and accurate over the past few years and allow large areas to be quickly mapped.

However, the main inconvenience of this technology is that the heat dissipation can vary according to a variety of aspects such as weather, light and erosion. Therefore, in order to locate a tunnel, one needs sufficient prior knowledge of the ground and its geological structure.

Microgravity
This solution sounds like it was taken right out of science fiction, and is based on measuring the earth’s gravity field in order to locate empty spaces in the ground. Gravity is affected by the mass of the ground, and the act of digging and moving ground around changes the mass. Even if we do not feel it, if you stand on top of a large tunnel, you will be a little bit lighter and lose a gram or two of your weight. In 2002, NASA launched several satellites in order to map the gravity field of earth so in the future we could create a geological map accurate enough to possibly prevent earthquakes.

But microgravity technology also has its defects, and is unable to differentiate between natural tunnels and man-made ones and requires years of study, which of course does not respond well to immediate needs out in the field.