In the skies over Gaza, Israel rules. Its planes, helicopters, and drones patrol and fire at will. On the ground, Israeli troops advance while Hamas lies in wait. But the ultimate battleground isn't visible from the sky or on your television news. It's underground.
Gaza is riddled with tunnels. Some are for smuggling; others are for transporting weapons; others are for hiding or ambushing Israeli troops. The crucial passageways—400 to 600, by recent estimates—run from Gaza to Egypt, circumventing the closed border. That's how Hamas gets parts and material for the missiles it fires into Israel. Any deal to end the current fighting has to include "an effective blockading" of that border, "with supervision and follow-ups," according to Israel's prime minister. To stop the war—and to keep it stopped—you have to figure out how to stop the tunnels.
But how? Here are some of the options.
1. Buffer zone. Israel used to control a 300-meter strip between Gaza and Egypt. That wasn't enough to stop Gazans from tunneling under it to Egypt. But what if the strip were thicker? Would that raise the cost of tunneling, or the probability of a collapse somewhere along the passage, enough to deter diggers? Israeli hawks want a buffer zone three kilometers thick, which would make tunnel excavation much more difficult, expensive, and time-consuming. Lately, the Israeli Defense Forces have dropped leaflets urging Gazans along the border to leave their homes—an attempt, some experts believe, to use the war to widen the buffer zone. But good luck getting Hamas, the Palestinian Authority, or European intermediaries to hand over three kilometers of south Gaza, much less remove the inconvenient residents from their homes.
2. Wall. Instead of thickening the old buffer zone, how about deepening it? Years ago, Israel tried a concrete-iron wall that extended 10 feet underground. A nice try, but fairly useless, since the tunnels went at least 20 feet underground. Then, just more than a year ago, two high-ranking officials from the U.S. Defense and State Departments went to Egypt with a proposal to build a new barrier, including "piles driven deep into the earth." But even if you extend a wall far enough underground, tunnelers can dig through it.
3. Moat. Maybe, instead of burying a solid barrier that could be dug through, we should make the barrier hollow and fill it with water. That way, anyone trying to dig through would—well, let's just say you wouldn't want to be there when it happened. This was such an intriguing idea that Israel tried it several years ago, soliciting bids for a moat four kilometers long, 100 meters wide, and 80 feet deep. Estimated cost: $250 million. Israel scrapped the plan because the water would come from the sea and might contaminate Gaza's groundwater. But the idea keeps coming back. Two years ago, Israel broached it again, and Egypt considered it. The U.S. officials who went to Egypt a year ago raised it again. Even the president of the Palestinian Authority has lobbied Egypt to do it.
4. Trench. If a moat is too dangerous to Gaza's groundwater, how about digging the same trench but leaving it empty? That would expose anyone who tried to get from one side to the other. Israel tried this idea, too, soliciting bids for a trench five kilometers long and 50 to 80 feet deep. The IDF even bought a 100-ton trench digger from Texas. The trench was supposed to be only 25 meters wide. But Israel dropped the plan because, at a minimum, it would have required demolition of 200 Palestinian homes. That's a problem, but less of a problem than the demolitions required for a buffer zone. And given the current alternative—smuggling, bombardment in Israel, and war in Gaza—everyone but Hamas might decide the demolitions are an acceptable price to end the fighting.
5. Ground-penetrating radar. If a barrier is too hard to build or can't do the job, maybe sensors can help. That's how the United States detects tunnels and digging along its border with Mexico. According to a presentation last month by the Army's Engineering, Research, and Development Center, we've used several methods: magnetic, electrical resistivity, ground penetrating radar, electromagnetic, and seismic. All of these methods involve sending waves into the ground and identifying anomalies on the rebound.
Some of the methods look unsuitable for Gaza. But what about ground-penetrating radar? This was a favorite tool along the Mexican border until tunnelers discovered its limits: It can't see deeper than one meter in wet dirt or 15 meters (49 feet) in sand, dry soil, or rock. At that point, all the tunnelers had to do was find the right terrain and dig under the range of GPR, making it obsolete. Good news: The ground around Gaza is dry and sandy. Bad news: Gaza's tunnels already go 50 to 60 feet deep. So GPR may not be up to the job.
6. Electromagnetic gradiometry. This might solve the depth problem. Originally developed for the demilitarized zone between North and South Korea, it detects underground voids by discerning slight anomalies in electromagnetic or gravitational fields. Companies that sell EM gradiometers try to keep the range secret, so they don't become obsolete like GPR. One published account estimates their outside range at 150 feet. That's deeper than any known Hamas tunnel. Still, it leaves the problem of administration. The IDF abandoned its strip on the Gaza-Egypt border four years ago because it was too hard to defend. Who's going to operate the machines?
7. Drone-operated gradiometry. Here's an idea: Put the tunnel sensors on unmanned aerial vehicles. Supposedly this has been tried successfully at least once on the U.S.-Mexico border. A year ago, the Department of Homeland Security told Congress that DHS was "experimenting with UAV mounted digital electromagnetic gradiometers." A presentation from the DHS Science and Technology Directorate depicts a team of drones (see Slide 27) using gradiometry to sniff out tunnels. The drones selected for the assignment are already available, "fully autonomous," can fly for 10 hours, and have "a data link range of up to 22 nautical miles." Or the IDF could modify its own drones to do the job. So Israel wouldn't need sitting-duck ground forces to monitor tunnels and diggers. It could hunt them from the air.
8. Automatic sensors. If you don't want drones along the border, you could try "acoustic" or "seismic" sensors. These require no operators and, according to a research paper that accompanied last month's Army presentation, can detect digging or movement in a tunnel even in conditions that "confounded GPR and electromagnetic techniques." The Army has field-tested a network of buried acoustic sensors in Iraq, with "overwhelming success," the paper reports. This network, which the Army now calls the Tunnel Activity Detection System, consists of buried sensors ("geophones") that are connected by an underground cable and transmit data to an operations center "via a satellite uplink." Theoretically, the geophones could be buried along the Egypt-Gaza border, and the operations center could be in Tel Aviv.
The U.S. Army Corps of Engineers is already working with Egypt on such a system. Recently, the United States allocated $23 million to Egypt for tunnel sensors. Two months ago, Ha'aretz reported that the corps was teaching Egyptian soldiers how to find tunnels using "instruments that measure ground fluctuations." Last week, the Washington Post said the corps was helping Egypt find tunnels with "sonar equipment." Apparently, what worked in Iraq is now being tried to Gaza.
Unfortunately, Israel doesn't trust Egypt to police the tunnels. Could Israel's defense industry build a similar system? It already has. Sonic Lynx, a firm based near Tel Aviv, advertises "an array of seismic and acoustic sensors deployed in the ground" that relay data "to a remote control and display station, where security personnel can view the classification of the threat together with its accurate location." Meanwhile, Electro-Optics Research and Development, a Haifa company that specializes in acoustics and seismology, has developed seismic antennas that can identify underground threats. Sonic Lynx recently lobbied the IDF to put its sensors under the Israel-Gaza border. In fact, Israel already has experience using acoustic sensors to hunt tunnels along Gaza's border with Egypt.
9. Statistical bombing. Having failed to block the Egypt-Gaza tunnels, Israel is now bombing them from the air and shelling them from the sea. Some tunnels were picked out beforehand—the Israeli Air Force hit 40 in a single night—but in other cases, according to Yedioth Ahronoth, the IAF "dropped at 10-meter intervals 600 kg bombs with timing devices, which 'statistically' hunt the hidden tunnels." If Israel can't get a deal to block the tunnels with sensors or a barrier, it might have to resort to "statistical" bombing again. That could mean a bombing campaign along the border every three to six months—the length of time it takes diggers to complete new tunnels. An ugly prospect, to be sure. But not as ugly as what's going on right now in Gaza.
(Human Nature thanks Slate interns Jennifer Akchin and Gage Newman, who tunneled through the Internet to bring back the parts that were assembled to make this article.)