Fifty feet underground, the tunnel looked like a giant wormhole: large enough for a man to crawl through, worn smooth by constant use and disappearing into the subterranean darkness beneath the Gaza-Egypt border.

Waist high, three feet wide and equipped with a motorized winch and electric lights, it was one of scores of Palestinian tunnels beneath the southern Gaza town of Rafah in March 2008, when a reporter visited.

Tunnels like those are now a principal focus of Israel’s military operation in the Gaza Strip, with bombing raids clearly audible on the other side of the eight-mile border with Egypt.

The 1.5 million Palestinians in Gaza regard the tunnels as a vital lifeline to the outside world, from which they are otherwise almost completely shut off by the Israeli military’s control of land, sea and air access to the north, east and west of Gaza. To the south, Gaza has been sealed off by Egypt.

In addition to the smuggling of everyday goods, tunnels like this, and others much larger and more sophisticated, are being used by Hamas to smuggle rockets and other weapons into Gaza from the Egyptian Sinai, and to move fighters in and out, Israel says.

Abu Qusay, the almost certainly false name of the tunneler who granted access to one of his routes, said that the tunnels took three to six months to dig by hand and machine, and could stretch for half a mile.

He said that the profits were divided among 10 partners — tunnel owners, builders, gatekeepers and smugglers — and that charges were high because the hole might operate for a year, or only a day, depending on when it collapsed or when the Israelis found it.

He said that before Israel left Gaza in 2005, about 95 percent of what the tunnelers brought in was weapons and ammunition. The most profitable items used to be bullets, for which smugglers charged around $5 each, and Kalashnikov machine guns, he said, adding, “We made a lot of profit from whoever paid.”

That pattern changed after Hamas forced its main rivals, Fatah, out of Gaza in 2007, Abu Qusay said, because so many weapons were coming in through the holes run by Hamas and other Palestinian armed groups that prices dropped, slashing profits. Since then, most of the ordinary tunnelers’ business, he said, was in everyday goods like electronics, cigarettes, cheese and infant formula.

“The factions have their own tunnels, so they bring in all their money, banned personnel and whatever weapons they can,” he said. “I don’t know anything about them. There is cooperation from Hamas, they give the tunnelers total freedom if we don’t bring in weapons or drugs.”

From the very outset of the Israeli military operation on Dec. 27, Israel identified the Rafah arms-smuggling tunnels as among the principal targets for airstrikes. The tunnels have also been shelled by Israeli gunboats, stationed miles away off Gaza’s coast to enforce a blockade.

An Israeli military spokesman in Tel Aviv said Friday that “dozens of tunnels” had been attacked and destroyed by the bombing raids. But he said Israeli forces were also facing an extensive tunnel network in Gaza City and elsewhere across the Gaza Strip, built to help fighters ambush Israeli troops.

The tunnels played a major role in the last round of heavy fighting, in summer 2006. Hamas burrowed a tunnel underneath Israel’s border fence to mount a surprise raid. The group’s fighters captured an Israeli corporal, Gilad Shalit, who remains a prisoner.

Brig. Gen. Yossi Kuperwasser, a former director of intelligence analysis for the Israeli military, said that since the Israeli pullout from Gaza in 2005, the number and size of the tunnels had grown immensely. That has allowed Hamas to smuggle in more and bigger weapons, including long-range rockets.

“We cannot afford to let the tunnels continue,” he said in a telephone interview. “We want to create a different security situation around the Gaza Strip. In order to do that we need to make sure that we can fully put an end to the entrance of weapons.”