October 6, 2011: The U.S. has suddenly sped up long-delayed shipments of deep penetrating bombs to Israel. This is mainly a political move (to get more votes from pro-Israel Americans). The original delays of those shipments were also political (to discourage Israel from attacking Iranian nuclear weapons facilities).
It all began seven years ago, in a $319 million deal that had Israel buying some 5,000 smart bombs from the United States. Included in the sale were 500 penetrating “bunker buster” bombs. These could be used against Iran's underground nuclear facilities, or the large number of underground facilities Syria has built over the years. The 500 BLU-109 bunker busters can penetrate five meters (16 feet) of concrete. The deal also included 2,500 909 kg (2,000 pound) bombs, a thousand 455 kg (1,000 pound) bombs and 500 227 kg (500 pound) bombs. The deliveries, especially of the bunker busters, were slow.
Before delivery was complete, there was a war with Hezbollah in 2006. At that point the Israelis really noticed the lack of the bunker busters and urged the U.S. to speed up delivery. More, but not all, of those bombs were quickly flown in. Israel already had several hundred of the GBU-24 penetrators (of about ten meters of earth), but a year earlier had ordered a hundred of the larger (2.5 ton) GBU-28s. This bomb can penetrate 32 meters (100 feet) of earth, or six meters (20 feet) of concrete. The lighter GBU-24 can manage less than half that.
Israel had watched (from the air, and via spies on the ground) as Hezbollah used lots of its Iranian money to build underground bunkers in the areas of southern Lebanon that Israel withdrew from in 2000. Hezbollah knew about the capabilities of the GBU-24 and 28, and built accordingly. That doesn't make the Hezbollah bunkers invulnerable. The entrances can be destroyed, and if you can get all the access tunnels, you turn the bunker into a tomb. But with some of the bunkers, not all the access tunnels were known. There's only so much that spies and air reconnaissance will tell you.