The senior official, who cannot be identified under the ground rules of the briefing, said the Israeli military had considerable prior knowledge of Hamas weaponry and tunnels, but was still "surprised" by the extent of both when the current ground operation began last Thursday.
He said the Hamas arsenal has now been revealed to include R-160 Iranian missiles with a range of 250 kilometres that were smuggled into Gaza in pieces and assembled there.
Even after launching by Hamas of more than 2,000 rockets already, the Israelis estimate that thousands more remain from an original supply of 6,000 in the hands of Hamas itself and another 4,000 in the hands of the hardline militia, Islamic Jihad.
Those include the heavy M-302 rocket with a range of 160 kilometres, enough to hit most of Israel, the official said, adding that it had captured and destroyed many of those before the recent conflict erupted.
Many other rockets, he added, were destroyed by the Israeli Iron Dome missile defence system. But he said smaller missiles, despite having smaller payloads, were harder to hit because of their size and limited time in the air. These include the M-75, with a range of 75 kilometres, the Grad, which reaches 45 kilometres, and the homemade Qassam rocket, with a range of just 17 kilometres.
Tunnels: 'a big achievement'
The Israeli defence official said the Hamas tunnel network was particularly surprising — both in its extent and in its sophistication.
"It's an industry that built, beneath Gaza, another terror city."
He said some 30 tunnels had been discovered so far, with a total of 66 entry and exit shafts and with many of those exits inside Israel. Roughly half have been destroyed so far. Even as recently as the last Israeli operation in Gaza, Operation Cast Lead in 2008, only small tunnels were found, the official said. Now, they're a highly developed strategic threat.
"Food, accommodations, storage, resupply — it's all happening underground.… It's a big achievement."
He added that the network had not been detected by aerial surveillance, because Hamas has solved the most obvious problem: how to hide the piles of sand removed from the tunnels. This, he said, was painstakingly taken away, a few bags at a time, and stored out of sight in buildings and underneath greenhouses.
The official said Hamas had diverted huge quantities of cement, imported for civilian construction, into the building of concrete-block walls and roofs for the tunnels.
As an example, he cited a 1.7-kilometre tunnel discovered in February 2013 that Israeli engineers estimated would have required 500 tonnes of concrete. "Enough to build a three-storey hospital."
More recent tunnels are even longer, running as long as three kilometres inside Israel and each with a height of 1.8 metres, enabling a fully armed fighter to move with ease.
'We have luck, I think'
The official said his country had been fortunate that Hamas had not taken a chance, before the Israeli invasion of Gaza, to exploit the tunnels for attacks inside Israel. Asked why that might be, he said, "We have luck, I think," but speculated that Hamas may not have been ready for the next step in a larger and more ominous strategic design.
He painted what he called an "apocalyptic" scenario where Hamas and its backers in Qatar and Iran might be capable of co-ordinated attacks with Hezbollah, the Lebanese Shia militia, which he said has also stockpiled rockets on Israel's northern border. He said he did not believe Hezbollah has a comparable tunnel network.
However, he added that "the West cannot tolerate" the threat posed by a Hamas with advanced weaponry and powerful allies — becoming less like a terrorist group and more like a terrorist state.
In closing the briefing, the senior defence official jabbed his finger toward his final PowerPoint graphic, projected on the wall.
"I'm not saying civilian casualties should not be covered, not at all, but that's the story."
The graphic said simply, "State-Like Capabilities."
He said it will be a political decision, not a military one, whether to put an end to the Gaza operation before those capabilities are fully eliminated.