Israel and Hamas are negotiating a four-day ceasefire which could see the release of up to 50 prisoners being held in Gaza as well as Palestinians currently imprisoned in Israel. Israel said that for every additional 10 hostages freed by Hamas, the truce would be extended by a day.

It is widely believed that hostages in Gaza are being held in a system of tunnels underneath the territory. This subterranean network is believed to have existed under Gaza. But how big and connected the tunnels are remains still unclear.

Drew Craig is a consultant geologist and a member of the International Working Group on subterranean warfare, based in England. He’s on the show to tell us what we know about the tunnels and how it’s made the strategy of this war even more complicated.

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