As the next phase of the war gets underway, Israel targets Hamas' 300-mile tunnel network beneath Gaza.

Israel has started to cross the border and enter Palestinian territory with troops and armored vehicles, marking the start of the next phase of its offensive in the Gaza Strip.

However, a large portion of the conflict between Israel and Hamas, the organization in charge of Gaza, may take place underground rather than on the city's streets, where Hamas is thought to have constructed a sophisticated network of tunnels and where the militant organization is also thought to be holding hostages.

In an effort to destroy Hamas in the maze-like network of tunnels, shafts, and chambers that is thought to extend more than 300 miles and possibly deeper than 200 feet, Israel claims to be striking hundreds of subterranean targets. Known as the "Gaza Metro," Israel asserts that Hamas organizes and executes its attacks out of this subterranean labyrinth.

Joel Raskin, a scholar who has spent more than 50 years studying the development of Gaza's tunnel system, stated that "they're pivotal for anything that Hamas has planned to do."

Early narrow tunnels were dug by hand with simple tools, and were used to smuggle goods in from Egypt's border. They were later turned into weapons. These days, the tunnels are practically undetectable and have been updated for attack, including phone lines, electricity, and concrete reinforcement.

According to Raskin, a professor of geomorphology at Bar-Ilan University in Israel, "the geology of the Gaza Strip is ideal for tunnel digging and maintenance, but it's very complex for tunnel detection based on the abundant layers of sediment."

According to the Israeli army, eliminating Hamas entails eliminating the network of tunnels, which includes attack shafts close to the border between Israel and Gaza, defense shafts farther back, underground artillery pads, and escape tunnels connected to apartment buildings and hospitals.

a statement released by Israel's top military spokesman, Daniel Hagari, Hamas maintains operations within and beneath Shifa Hospital, the biggest hospital in Gaza, as well as other medical facilities within the region.

Despite Israel's increasing ground operations, Hamas disputes the existence of tunnels beneath the Shifa hospital, which claims to be providing shelter to 40,000 displaced Palestinians and medical care to the injured and defenseless.

Israeli reserve colonel Amir Ulo entered a Gaza tunnel for the first time in 2007. Since then, the U.S. Army Corps of Engineers has constructed mock tunnels for Israel's military to use for training.

Israel has been bombing Hamas' networks for the last three days in an effort to breach them, even though foreign hostages are being held captive there.

"I'm not telling you that we are not going to face losses," Ulo stated. "We don't want to go to war. We are looking for harmony. But we know how to fight when the time comes for combat. We will carry it out. And we are going to win."

Benjamin Netanyahu, the prime minister of Israel, has described the conflict as a struggle for Israel's survival. However, the families of those held captive fear their loved ones won't survive.