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Holey WarHow to close the Gaza tunnels.

By William SaletanPosted Friday, Jan. 16, 2009, at 7:58 AM ET

Masked Palestinian men lower a cow into a underground chamber 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.

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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.

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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.