Since Hamas took control of Gaza two years ago, when they defeated Fatah (militarily, and at the ballot box), they have found out how to make money from misery. First, they had to turn Gaza into a police state. Then they took charge of the economy by controlling what goes through smuggling tunnels, and what people pay for it. While Hamas complains about the Israeli blockade, which is in place because Hamas refuses to stop demanding that Israel be destroyed, and bringing weapons (especially long range rockets) into Gaza to make that happen, they would go bankrupt if the blockade were lifted.
These smuggling operations are so lucrative that Hamas deems them legal enterprises, and charges a $2,500 fee for anyone who wants to build and operate a tunnel. In addition, armed Hamas revenue collectors stand at the Gaza entrance for each tunnel, demanding a payment for everything coming out of Egypt, or going there.
Exactly how many tunnels there are is something of a mystery. Given that the Gaza border under Egyptian control is only about 14 kilometers long, it would appear that the actual number of operating tunnels is closer to a hundred, or less. But Israeli sources frequently say there are 300 or more.
Building and using tunnels under the Egyptian border is a big business, employing over 5,000 Palestinians. During the last two decades, frequent outbreaks of Palestinian terrorism have led to the border being closed. A security fence between Gaza and Egypt made above-ground smuggling difficult, so the Palestinians began digging tunnels, and using them to smuggle people and goods into, or out of, Gaza.
Israel and Egypt are cooperating to identify the tunnels used to move missiles and other weapons into Gaza. Egypt passes information on to Israel, and then these tunnels are shut down with an Israeli air strike (sometimes using a penetrating bomb, if needed). But most of the tunnels are left alone, so consumer goods can get into Gaza, and the Egyptian border force can continue to extort bribes from the tunnel operators. Egypt is keen on shutting down the weapons shipments because Hamas is also sheltering Islamic terrorists who focus their attacks on Egyptian targets.
The 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. Thus to build 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.
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 these days. But the potential profits are enormous. Currently, moving a person through a tunnel costs several hundred dollars, or more. A sack full of goods, or a jerry can of fuel, costs several times its value to move through the tunnel.
The downside is that most tunnels 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 (they left four years ago), they got pretty good at finding, and destroying, tunnels. The Egyptians, who now guard the border, are nowhere near as good, and they can be bribed. But even today, a tunnel rarely lasts more than a few months, and someone usually dies as it collapses and goes 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.