Six months ago, the Palestinian militant group emerged battered and bruised from a 50-day conflict with Israel, where it fought alongside fellow Islamist movement Hamas, which controls Gaza.
Islamic Jihad's armed wing the Al-Quds Brigades lost 123 men, but fired 3,249 rockets at the Jewish state, it says, including Iranian-made Fajr 5 missiles which reached Tel Aviv and even northern Israel.
The conflict, where Israel tried to flush out Hamas and Islamic Jihad militants by bombarding the coastal enclave, killed nearly 2,200 Palestinians, mostly civilians, and 73 people on the Israeli side, mostly soldiers.
But the group says it is resurgent.
"We're at our highest level of alert to counter any attack. We're used to the occupation (Israel) breaking its ceasefires," senior commander Abu al-Bara told AFP from one of the group's tunnels.
"What doesn't kill us makes us stronger. We're gathering all our strength since it's a war that never ends. We're ready to go to another level against the Zionist occupation and carry out actions we've never done before," he said, without elaborating.
The war ended on August 26 with a fragile Hamas-Israel ceasefire brokered by Egypt.
In the meantime, Islamic Jihad has sought to replace its fallen comrades, and its manpower is estimated at between 10,000 and 15,000 fighters.
Hamas and Islamic Jihad refuse to specify the number of fighters, or rockets, they currently possess, but both were severely depleted by the Israeli assault, which included fierce ground combat.
- 'Abu Bakr street' -
Israel's ground invasion was aimed at destroying the hundreds of tunnels Gaza's two main Islamist groups had built, both for smuggling supplies and for attacks inside Israel.
Islamic Jihad and Hamas staged deadly assaults on Israeli troops using the tunnels.
For the first time, AFP correspondents were permitted to enter a tunnel, provided they were blindfolded whilst being taken to them, so as not to give away its location.
Inside, placards had been erected on the concrete walls, naming "streets" to help militants navigate the network.
One route was named after Abu Bakr al-Sadiq, the first Islamic caliph following the Prophet Mohammed's death in 632; another, Othman Ibn Affan, a successor.
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Quds fighters said the group had a network of tunnels of "different lengths, some with several entrances".
But they were silent on the network's location or how it was built.
Their weapons were made inside Gaza, they said, but without elaborating on how the parts were obtained or purchased.
It was an open secret before July 2013 that smuggling tunnels underneath the Egyptian border brought in much of Gaza's construction and fuel needs, money, and weapons, or the material to make them.
But Egypt destroyed the vast majority after the military ousted Islamist president Mohamed Morsi and a crackdown was launched against his Muslim Brotherhood, Hamas's ally.
Islamic Jihad, founded like Hamas in the 1980s, traditionally has its closest ties with Tehran, having been inspired by the Iranian revolution.
- 'Mujahedin school' -
Israel accuses the groups of continuing to seize construction material entering Gaza to make tunnels and weapons.
"Hamas and the Palestinian Islamic Jihad have openly declared that they are rebuilding tunnels and testing rockets and other things like that," army spokesman Lieutenant Colonel Peter Lerner told AFP.
"They've been siphoning off from the civilians of Gaza forever," he said, warning that Israel was prepared for further confrontation.
Underground and overground, the Quds Brigades lie ready.
In the southern city of Khan Yunis, a banner reads: "School of the Mujahedin".
Some 200 new recruits aged 19 to 22 are training at the camp, ducking under live fire, jumping through flaming hoops and shouting "Death to Israel! Death to America!".
Instructor Abu Ahmed says the masked young men are going through "very advanced training".
"Their training lasts between 36 days and six months, before they go to a specialised unit like explosives or sharp shooting," he said.
"We don't just have tunnels -- we also invest in people," Abu al-Bara interjected.
"With ideas and fighting, it is people who will liberate this land."