Hamas's underground combat manual, seized by Israeli forces and reviewed by The New York Times, details the meticulous planning and investment in building an extensive network of tunnels beneath Gaza. The manual includes instructions for navigating in darkness, using automatic weapons, and fortifying the tunnels against airstrikes and ground assaults. Hamas has invested heavily in this underground infrastructure, which has proven to be more sophisticated and extensive than initially estimated by Israeli officials. The tunnels have been used for communication, command, and control, and to launch hit-and-run attacks, making it difficult for the Israeli military to dismantle Hamas's operations. The extensive tunnel network has slowed Israel's assault, leading to a prolonged war. Destroying the tunnels is a time-consuming and resource-intensive task, impacting the pace of military operations. The tunnels have also been used to hold Israeli hostages and have become a central target for Israel's military leadership. The cost of building and fortifying the tunnels has been significant, reflecting the strategic importance of this underground infrastructure for Hamas. Israeli officials have encountered blast doors and other fortifications inside the tunnels, indicating a sophisticated and well-prepared underground defense strategy by Hamas.