Whenever they hear the sound of Israeli warplanes, the young men emerge from holes, run away quickly and stand watching from a distance.

They go back to their work after the jets drop several bombs alongside the Palestinian side of the border with Egypt in a bid to destroy hundreds of tunnels used to smuggle goods into the besieged Gaza Strip.

Israel started to attack the smuggling tunnels when it launched the major offensive all over Gaza Strip in December and January. Though the Israeli intensive operation ended, the air strikes at against the underground tunnels continued almost every day.

Mahmoud, a high school student in his early twenties, was standing watchfully on a small hill, ready to warn his colleagues.

"The lives of tens of my colleagues are subject to my alertness; I warn them about everything even if the sound of the planes are far away," he said.

Mahmoud spends six hours a day as a guard then another friend takes on the job in shifts. "Someone must stay watching because those who work in the tunnel can not hear the sound of the airplanes due to the sound of the dragging and digging machines they use under the ground," Mahmoud said.

In the past, the smuggling process used to be done at night while the workers spend the day in digging and expanding the underground passages. But due to the recent threats, they stopped working at night.

"It is more dangerous these days, sometimes their (Israeli) airplanes fire the rockets from a distance and surprise us," said Yasser, another young worker.

"What was used to be brought in few hours through the tunnel is now taking days to be received here," he adds.

The tunnels are mainly used to bring in fuel, food, spare parts, computers, cell phones and all household items to the enclave from neighboring Egypt.

The tunnel business flourished since Israel tightened its blockade on Gaza Strip in 2007 in a bid to weaken Hamas, the Islamic movement which controls the territory. Hamas legitimized the smuggling to ease public pressure on it.

Israeli attacks on the tunnels took toll on the prices of almost everything in the markets as the flow of goods through the narrow dark alleys is decreasing due to the increasing risk.

Sameer Qadoura, an owner of a supermarket, said the prices of the tunnel-imported stuff have jumped during the height of the war but decreased again after the war ended and the people found out that most of the tunnels are still working. "They dramatically jumped but now the average of the increase in the prices varies around 20 percent."

Walking in the supermarket and pointing at chocolates and cleaning materials on the shelves, Qadoura says 80 percent of the products in his store are brought through the tunnels. "What we officially get from Israel is dairy products and some frozen meat."

Over the past months, the tunnels resolved the crisis of transportation fuel after the smugglers successfully pumped thousands of gallons to Gaza Strip, causing the price of the one liter of diesel to go down from 20 Shekels to 1.7 Shekels. The prices of the diesel are gradually rising again, standing now at about four Shekels per liter.