The tunnels of Củ Chi are a network of connecting underground tunnels used during the Vietnam war. The locals would live under group and defend their land.
I booked the tour via TheSinhTourist at the same time as booking onward tickets to Cambodia. The Củ Chi Tunnels cost 109,000VND for half a day with a tour guide and entry.
It is really useful having a guide explain to you all the details about the tunnels, traps and history of what happened.
I crawled the tunnels, and when I popped out the other side my entire tour group including Kim had disappeared.