Nick Schifrin:

They emerge from the darkness into the light only long enough to strike. Hamas propaganda videos show fighters exiting tunnels to fire at nearby Israeli tanks.

These are guerrilla tactics, small Hamas teams sneaking up on Israeli soldiers with far superior technology, or using drones to drop mortars, helping make Israel's Gaza ground invasion deadly and difficult. Buried deep underground, Hamas has built what Israel assesses is a tunnel network 200 to 300 miles' long. Dug by hand, it's called the Metro, where Hamas fighters can move weapons and themselves safely.

The tunnels have ventilation, electricity, and are Hamas' single most important asset.

Maj. John Spencer (Ret.), Modern War Institute: Hamas has built for decades tunnels leading under, integrated in to all the Gaza cities.

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