Just after midnight on Tuesday, the Israel Defense Forces announced that they had struck a tunnel that allowed Hamas to “infiltrate Israel through the sea.” The sea tunnel was unusual, indicating that Hamas has developed new and deadly methods to attack Israel. Hamas has an extensive network of tunnels beneath the Gaza Strip, often described as “miniature cities,” with this particular tunnel having an exit on a beach.

The tunnels pose a significant challenge for Israeli forces, which is why they delayed sending troops into Gaza after the recent Hamas attack. Hamas has spent years refining its tunnel system, which includes subterranean pathways, rooms, cells, and even underground roads. These tunnels serve as hiding places for weapons, fighters, and command centers.

Overnight on Saturday, Israeli fighter planes targeted 150 underground locations in the northern Gaza Strip, according to the Israeli military.

For Israeli forces participating in a large-scale ground invasion, the tunnels will be one of their most formidable challenges. General Joseph L. Votel, the former leader of United States Central Command, warns that the fighting will be brutal and bloody.

Tunnels have been a part of life in Gaza for years, but their numbers increased significantly after Hamas took control in 2007 and Israel imposed its blockade. Palestinians used the tunnels to smuggle in food, goods, people, and weapons. Hamas invested approximately $3 million in each tunnel, constructing them with prefabricated materials and including features like medical rooms and hiding spaces.

The tunnel system in Gaza is often referred to as “lower Gaza” or the “metro” by Israelis. The tunnels stretch all the way to the Israeli border in the north. Some tunnels have been discovered under Al Shifa Hospital in Gaza, the largest medical center in the territory, which has raised concerns about the use of civilian infrastructure for military purposes.

Hamas concealed tunnel entrances in houses and small structures on the Egyptian side of the border, enabling the smuggling of goods. However, Egypt later destroyed many tunnels along its border, including dumping sewage into some and leveling houses with concealed entrances.

The exact length of Hamas’ tunnel network is unknown, with estimates ranging from hundreds to over 300 miles. The tunnels used for launching attacks on Israel are typically dug by hand or with shovels to avoid detection. These tunnels are around six and a half feet tall and three feet wide, making it challenging for soldiers to navigate through them.

Tunnels often have booby traps, making them dangerous for anyone attempting to enter. Remote airstrikes and sensors can destroy tunnels, but ultimately ground forces are required to ensure the complete dismantling of the network.

The Israeli military has specialized units and training programs focused on underground warfare, including identifying tunnels and destroying them. Despite these efforts, the tunnels remain a persistent challenge for Israeli forces.