The level of smuggling tunnels between the Gaza Strip and Egypt now amounts to an industry, according to UN officials.
A UN report said the tunnels had become a lifeline for Gaza's Palestinians because of the Israeli blockade. Thousands work in the tunnels.
The tunnels are used to smuggle a wide variety of products into Gaza - including food and fuel.
But Israel says they are also used to import arms. It accuses Egypt of not doing enough to stop the smuggling.
Media reports estimate there are hundreds of tunnels in use along the border.
The UN report says that because of the inability of many businessmen to conduct trade through official Gaza crossings, such as Karni, Sufa and Kerem Shalom, the tunnels have become increasingly vital to Palestinians.
But they also pose a danger. About 40 people have been killed as a result of tunnel incidents since the start of the year.
Last month, the Hamas authorities which control Gaza introduced regulations to licence and control trade through the tunnels.
A list of conditions were announced, including a call for all tunnel operators to meet certain standards.
Numerous tents covering tunnel entrances are visible in the Rafah area, according to the UN report, and the industry is becoming increasingly open and controlled.
But the Israeli blockade is having an ever greater humanitarian impact on Gaza and the UN said the re-opening of the official crossings with Egypt remained a priority.