In addition to the smuggling routes on the Egyptian border that are crucial for the movement of imports, Gaza's subterranean system serves additional functions. Many of the tunnels provide passage under populated urban areas — to serve as a place where Hamas leaders and their weapons are shielded from potential air attacks — and on the eastern border, are designed for incursions into Israel.

Hemmed into a 139-square-mile territory, the 1.8 million Palestinians have become reliant since 2007 on goods that arrive underground from Egypt through the city of Rafah, despite efforts in the last years from successive Egyptian governments to close or destroy some of the tunnels. Beyond giving Hamas tax revenue and weapons, the tunnels supply high-demand civilian goods like food and medicine, as well as infrastructure materials including concrete and fuel.

With a land blockade on Gaza’s three sides enforced by Israel and Egypt, and a siege maintained by sea and in the air above its coastal strip, Gazans have found a way around their confinement by going down into the earth .

At their peak, the tunnels reportedly funneled some $700 million into Gaza’s economy and provided employment for as many as 7,000 people. The lucrative but perilous traffic is thought to continue today through about 500 tunnels .

The Gaza tunnels have been part of life since the 1980s, but their importance grew after Hamas routed Fatah , and Israel subsequently placed severe restrictions on the territory. The result was that more of Gaza’s underground trade shifted to the Egyptian border, and the Hamas-controlled Interior Ministry’s Tunnel Affairs Commission began to regulate commerce .

Dan Murphy, a Middle East correspondent for The Christian Science Monitor, said Hamas justifies its investment in the tunnels because the underground network is an “existential issue for the movement.”

Some of the tunnels originate near the Israeli border in the Gaza City suburb of Shujaiya, where bloody clashes on Sunday saw a sharply increased death toll on both sides .

Israel's invasion of Gaza was launched on the premise that Hamas tunnels could mostly be destroyed. Andrew Burton / Getty Images

Border tunnels into Israel

For Israel, the below-ground equivalent of the Iron Dome anti-missile system — tunnel-sensing seismic monitors and algorithms — is far from being deployed. Geologists argue that combating tunnels can be solved through technological innovation but are not “rocket science.”

One tunnel discovered by Israel last year was 66 feet deep and 1.5 miles long. The project is estimated to have cost $10 million and used 800 tons of concrete . The dangerous digging was apparently done with mechanical pedal-powered devices , rather than with noisy electrical equipment.

Hamas forces traveled in the tunnels to capture Israeli soldier Gilad Schalit in 2006 and bring him back to Gaza, where he was kept prisoner for five years.

“Tunnels are just one weapon used by the resistance ,” Abu Obeida, a Qassam spokesman, told Al Jazeera. “They can move from a defensive position to an offensive one in any situation.”

The counterterrorism unit of the Al Qassam Brigades , the armed wing of Hamas, has created secret spaces in tunnels where soldiers can spend more than a week. In a report that aired on Al Jazeera Arabic last year, a senior member of the group described how dangerous the tunneling job is — vulnerable to tunnel collapse and targeting by Israel.

By circumventing the eastern border, where there are buffer and no-go zones that are 2,000 feet wide and feature double-wire fencing with watch towers, the tunnels from Gaza into Israel are seen as a security threat to Israel.

Attempts to destroy the “attack tunnels” occurred in the run-up to war in late 2008, and Israel thought most of them were shut down in the subsequent Operation Cast Lead. But this month's Operation Protective Edge, now in its third week, has shown the resilience of Hamas' new tunnel strategy.

Murphy summarized the Israeli army’s "neutralization" task: “They can find the exits and work back, but there are fears of booby traps. If they really do see this as a serious threat, then they need to push people back from the [border] fence even more, so [tunnel diggers] would have to go farther distances.”

On July 6, violence intensified after an explosion killed six Hamas men in a tunnel. Then a tunnel infiltration by 13 gunmen headed to Kibbutz Sufa on July 17 preceded Israel’s ground invasion later in the day.

Tunnel infiltrations on July 19 included a raid by Hamas on a patrol jeep near Kibbutz Ein Hashlosha, and another near Kibbutz Be’eri, by fighters equipped with tranquilizers and handcuffs.

And on July 21, a incursion near Kibbutz Nir Am featured 10 Hamas men dressed in Israeli army uniforms but wielding Kalashnikov rifles, which are not used by the Israel Defence Forces (IDF).

Israel says in its current Gaza incursion it has uncovered 66 entrances to some two dozen tunnels. Engineering Corps demolition teams use controlled explosions to destroy tunnels that often contain communications lines and barrel bombs, and Israel says it has fully destroyed six of the passages that crossed the 25-mile frontier.

Gaza’s internal tunnel network is reportedly even more complex than cross-border routes and involves multiple branches that run under refugee camps in Khan Younis, Jabaliya, Shati and other densely populated areas. These hide weaponry and are designed for Hamas leadership to remain protected and mobile.

However, much of the present combat will continue over the cross-border tunnels. Raw footage from Hamas released by The Associated Press shows Hamas fighters marching through a sophisticated tunnel. A video released by the IDF shows soldiers reportedly blowing up tunnels they discovered.

Another clip has troops explaining the operation, and pointing out some of the accommodations constructed by Hamas tunnel crews.

“Hamas wants to get a little something in terms of relief, letting goods and services flow," said Robert Hunter, former White House representative to Arab-Israeli peace talks. But, he added, "Israel probably won’t stop until they destroy most or all of the [attack] tunnels.”