JERUSALEM (AP) -- A network of tunnels Palestinian militants have dug from Gaza to Israel -- dubbed "lower Gaza" by the Israeli military -- is taking center stage in the latest war between Hamas and Israel.
Gaza's Hamas rulers view them as a military game changer in its conflict with Israel. The Israeli military says the tunnels pose a serious threat and that destroying the sophisticated underground network is a key objective of its invasion of Gaza.
Israel has known about the tunnels for several years, but has been hard-pressed to find an effective way to block them. Now it is counting on its ground war to at least reduce the threat.
"Israel knew there was a problem with the tunnels, but it didn't internalize their significance," said Shlomo Brom, a retired Israeli general. "At any given moment, Hamas could send dozens of militants through separate tunnels to attack communities in Israel."
Gaza has two sets of tunnels -- those reaching Egypt and those reaching Israel.
The underground passages to Egypt are meant to bypass a border blockade on Gaza that was tightened by Israel and Egypt after Hamas seized the territory in 2007. The tunnels provide an economic lifeline and are used to deliver building supplies, fuel, consumer goods and even cattle and cars.
In some of those tunnels, Gaza militants received weapons and cash from their patrons abroad, particularly Iran. Egypt has destroyed virtually all of the tunnels over the past year, driving Hamas -- which was taxing the smuggled imports -- into a severe financial crisis.
Separately, Hamas significantly expanded its network of tunnels from Gaza to Israel after a major Israeli ground offensive that ended in January 2009.
"There are thousands of resistance fighters working underground and thousands others working above the ground, to prepare for the upcoming battle," Ismail Haniyeh, a top Hamas leader in Gaza, said earlier this year. "No one can imagine what the resistance is ready to do to confront the occupiers."