Can the Israeli army win its war against Hamas without a large-scale invasion of the tunnels dug by the Islamist group beneath the Gaza Strip? This question has haunted Israel since the launch of its ground operation in the Palestinian enclave on October 26, 2023. "Half of the territory of North Gaza has been destroyed by Israeli bombardment, but the network of tunnels remains intact for the moment," said a French military source.

Until now, the Israeli army had estimated the extent of the system built under Gaza at around 400 kilometers. But its first underground incursions showed that the network of tunnels could in fact be twice as large – the equivalent of almost four times the size of the Paris metro. "The sheer size of Hamas’s underground networks may, once fully discovered, be beyond anything a modern military has ever faced," said John Spencer, the chair of urban warfare studies at the Modern War Institute, in an analysis piece published on January 18.

This labyrinth of bunkers and tunnels, equipped with water, electricity and mechanical or natural ventilation, serves as a refuge for Hamas, but also houses command posts, ammunition storage, weapons workshops and more. The 136 hostages still in the hands of Hamas (31 have been declared dead by Israel) are also believed to be being held there. Above all, this network poses a permanent threat to the Israeli army, which can never be sure that Palestinian fighters will not emerge from the ground in the territories it has conquered.

Since the launch of its Iron Swords operation, Israel has been using "bunker busting" bombs, in particular US-supplied GBU-28s, to reach the Palestinian network's tunnels and access shafts. But these are largely ineffective. "Destroying a concrete and iron shelter more than 20 metres below ground is virtually impossible from the outside," explained a French engineering officer, pointing out that "no buried Serbian command posts were destroyed during the war in the former Yugoslavia." According to the Israeli army, some Hamas underground tunnels are 70 or 80 meters deep.

To dislodge Palestinian fighters from the "Gaza metro," as this underground maze is known, various experts have recommended flooding the tunnels. On January 30, the Israeli army reported that it had begun sending "large volumes of water" into the tunnels in the south of the enclave. But the extent of the network is such that Western militaries doubt the effectiveness of such measures – not to mention the risk of contaminating the region's rare water tables if seawater were to be used.

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