Building tunnels under the Egyptian border has become a big business, employing over 15,000 Palestinians. 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. Hamas sees the tunnels as a long range operation, that will keep essential weapons and supplies coming, no matter how much Israel tried to isolate Gaza.
Meanwhile, another rocket from Gaza, was fired into Israel on July 16th. No one was injured, and the most notable aspect of this attack was that it was the first one in a month. Israel has made it clear that the blockade would not be lifted until the rocket attacks stop, and the flow of weapons into Gaza, especially longer range Iranian rockets, stops. Hamas has been cracking down on the other terrorist groups that keep trying to fire rockets into Israel, but refuses to halt the weapons shipments. Hamas is determined to destroy Israel, and the stockpiling of Iranian rockets is seen as essential to that. Peace talks are stalled over issues like this.
No one is sure how many smuggling tunnels there are under the Gaza-Egyptian border. 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 $5,000 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.