The "Gaza Metro" - The network of tunnels Hamas has built under Gaza

The “Gaza metro.” It’s a vast labyrinth of tunnels, by some accounts several kilometers underground, used to transport people and goods; to store rockets and ammunition caches; and house Hamas command and control centers, all away from the prying eyes of the IDF’s aircraft and surveillance drones.
Hamas in 2021 claimed to have built 500 kilometers (311 miles) worth of tunnels under Gaza, though it is unclear if that figure was accurate or posturing. If true, Hamas’ underground tunnels would be a little less than half the length of the New York City subway system.