It was more sprawling than deep. It was a series of trenches connecting "rooms". I know they also had "water features" at some points, but the water would soak into the ground pretty quickly then be a mess for a few days, so they didn't do that.
No collapses happened and everyone is still alive. :-)
Happy that all's well that ends well, but for any parents considering this, trench collapses have killed hundreds of workers in just the past decade. Anything deeper than a couple feet might be a hazard that needs to be mitigated.
Also depends on the local geology and how tenacious the kids are. I couldn't dig to dangerous depths as a kid even if I wanted to because the soil got way too rocky to dig through before 3 feet down.