Rafah, Gaza Strip: The owners of hundreds of tunnels, running under the Gaza-Egypt border, have registered with the Hamas authorities, signed pledges to pay workers' compensation and hooked up their underground operations to the local electricity network.

The once-clandestine business has come out into the open in recent months. Dozens of large tents, each marking a tunnel work site, have been pitched along the border, just yards from Egyptian watchtowers.

With Gaza's borders virtually sealed by Israel and Egypt for the past 16 months, the tunnels are now one of the territory's lifelines and are seen as key to keeping Hamas, the group of Islamic fighters, in power.

The underground imports - from refrigerators, food and clothes to fuel and anti-tank rockets - prevent Gaza's economy from collapsing, reduce the risk of food riots and help beef up Hamas's weapons arsenal.

"The tunnels have become the main source of commodities in Gaza and every day the closure continues, the importance of the tunnels increases as well," said Gaza economist Omar Shaba'an.

Israel has long complained that Egypt is turning a blind eye to the smuggling. Under US pressure, Egypt has been trying harder to clamp down.

Egypt says it has destroyed scores of tunnels since Hamas's violent takeover of Gaza in June 2007 and has stepped up the pace after getting $28 million (Dh102.98 million) of US detection equipment four months ago. But critics argue Egypt could do far more.

Smugglers say the demolitions disrupt business, but don't halt it. Most tunnels have several exits, like branches on a tree, and operators say they can quickly dig new openings.

Hamas is treading carefully, keeping the tunnels operating, but trying not to be too brazen so as not to provoke Egypt, a key link to the Arab world.