The United States recently sent a team of officers and NCOs to Israel to study what Israel had discovered about the Hamas use of tunnels in combat. The U.S. and Israel have been sharing intel like this for decades. The Americans gathered a lot of data on Taliban and Iraqi use of tunnels since 2001. Apparently Hamas knew about a lot of that (Islamic terrorists share their tricks with each other) and came up with some new ideas, which the Israelis got very intimate with during the recent (July-August) “50 Day War” in Gaza. There Hamas used a lot of tunnels, some of them into Israel but most of them were all within Gaza, or to Egypt. What concerned the Israelis most were the new tunnels (some of them over a thousand meters long) into Israel. These were also deeper than the usual smuggling tunnel, so they were harder to find and the exit on the Israeli side was not completed until just before Hamas was to use the tunnel to bring large numbers of terrorists in to kill or kidnap Israelis.
The Gaza area is a desert, and if you dig down 6-20 meters, you'll find hard sand that can be excavated at the rate of 15 meters a day. The smuggling tunnels into Egypt have been used for decades. To build such a tunnel you need cooperation from building owners on both sides of the border. They expect to get paid, usually a flat fee to start work, then monthly "rent", or even a percentage of revenue from people and goods going through the tunnel.
The Egypt tunnels tend to be 500-600 meters long. So including digging down on each end, it's going to take you 5-6 weeks to complete your tunnel. If you have a few experienced (and highly paid) people working with you, the whole project will cost you $25,000 or more. That's a lot of money in Gaza. But the potential profits are enormous. Before the 50 day war moving a person through a tunnel costs several hundred dollars, or more. A sack full of goods, or a 20 liter can of fuel, costs several times its value to move through the tunnel. Since the 50 day war Egypt has destroyed nearly all the smuggling tunnels into its territory and has cleared buildings and people from a 500 meter wide “security zone” on their side of the Gaza border. Hamas is now more isolated than ever.
The downside of these tunnels is that most are just wide enough (about a meter, and a little less tall) for a man to crawl. The air is foul and the risk of collapse is constant. Few tunnels are built with bracing, to prevent, or mitigate a collapse. It is believed that hundreds of Palestinians are dead and buried under the border, as a result of collapsed tunnels. When the Israelis ran the place (until they left in 2005 as a peace gesture that backfired), they got pretty good at finding, and destroying, tunnels. The Egyptians, who now guard the border alone were nowhere near as good, and could be bribed. But even before the 50 day war a tunnel rarely lasted more than a few months, and someone usually died as it collapsed and went out of service. Thus the high fees for getting stuff through. The men who move goods through the tunnels are highly paid, but are poor insurance risks. Since the Egyptian borer is totally closed now, bribes are much less effective and there is more economic incentive than ever to build tunnels into Egypt. These will be extremely expensive because the Egyptians are also digging a 20 meter (62 foot) deep canal from the Mediterranean along the Gaza border. In addition to slowing down smugglers or infiltrators, this will make tunnel building more difficult and dangerous.
Over the years Hamas has learned how to dig tunnels that are virtually undetectable on the Israeli side. This means going deep enough to avoid detection by ground penetrating radar or acoustic sensors. This makes it more expensive and time consuming to build tunnels but Hamas has diverted much foreign aid (cash and building materials) to the tunnel effort. A lot of the concrete and other building materials allowed into Gaza went to building tunnels.
By 2014 Israel knew of the new, deeper and more complex Hamas tunnels into Israel but had found only four of them since 2012. In March Israeli troops found one that was 1,800 meters long and extended 300 meters into Israel. Hamas dismissed this find as a tunnel that had been abandoned because of a partial collapse. But the Israelis said the tunnel had been worked on recently and equipment, like generators, was found in it. The tunnel was lined with reinforcing concrete and was 9-20 meters (30-63 feet) underground. Three of these tunnels were near the town of Khan Younis and apparently part of a plan to kidnap Israelis for use in trades (for prisoner or whatever) with Israel.
Israeli intelligence knew Hamas leaders had been discussing a much larger tunnel program, involving dozens of tunnels and suspected that dozens of them were already built or nearing completion before the 50 day war. Most of the completed ones had no exits in Israel, yet. Available monitoring equipment was slow and often ineffective if there was no one actively working on the tunnel below or if there was no exit (yet) on the Israeli side. Another problem was that Hamas had, by trial and error, discovered the limitations or blind spots of Israeli tunnel detection gear and techniques. Since the 50 day war the Israelis have discovered what their detection weaknesses are.
The Hamas tunnels already built and “stockpiled” before July could only be detected inside Gaza, where their entrances were. These were also hidden, at least from aerial observation. Israeli intelligence had discovered possible tunnel entrances by detecting the Hamas activity around the suspected entrances (entering and leaving, removing dirt). Hamas tried to hide this activity and Israel knew this meant they probably succeeded in some cases. Thus before the Israeli troops went into Gaza recently, commanders had lots of information of where to look. By the end of July the Israelis had found 46 entrances to 14 tunnels. They also discovered that many of the tunnels had numerous branches inside Gaza and were meant to be used to move Hamas gunmen and suicide bombers around if the Israelis invaded. These Hamas men could then come to the surface behind Israeli troops and attack. This was one reason why there were so many Israeli casualties this time around (compared to the 2009 invasion).
Israeli combat engineers had been trained to destroy the tunnels, which was not easy because Hamas had booby-trapped some of them. Israel eventually found and destroyed 34 of these tunnels and stayed inside Gaza until they felt they has found all of them, and collected information on how they are built and how they can be detected from the ground or air. If Israel knows where a tunnel is, before they destroy it they can run some tests with their sensors and that knowledge will make it more difficult for Hamas to build new tunnels. The U.S. and Israel are both working on new and better ways to discover these tunnels.