DARK, TERRIFYING, CLAUSTROPHOBIC - IDF Tunnel Unit fights Hamas

The Hamas tunnels: a wildcard in the Gaza fighting.

"It's very dark and it's claustrophobic and it's narrow. And you lose your sense of direction immediately. You lose completely a sense of time," said Daphne Richemond-Barak, an Israeli military analyst and the author of Underground Warfare. The Israeli military has taken her into Hamas tunnels that crossed from Gaza into southern Israel.

"They are humid and suffocating. So when you walk in them, you would be disoriented for sure, but also incredibly afraid," she added.

Hamas claims to have 300 miles of tunnels in Gaza, a subterranean complex that effectively serves as an all-purpose military compound. According to Israel, the underground space includes military headquarters, sleeping quarters, as well as workshops to make and store rockets.