I think the “primal urge” to dig is just really seeking the endorphins of manual labor. Digging like that is especially attractive because there’s little planning (unless you’re making a tunnel like the subject here) and no material investment but the earth beneath your feet.
One of my sisters had four boys (and no girls) and during summers they would drive her crazy with their boredom. When they were about ages 8-14 one summer she said: go in the back yard and see how big of a hole you can dig.
Wide-eyed they said: really? She said yes, dig as much as you want, but the only rule is it all gets filled in before school starts in the fall. 30 years later they say it was the best summer ever. Every day they were working on it and all of their friends would come by and help dig and plan what development would come next.
How deep did they get? Hope she kept an eye on it, unsupported holes quickly get dangerous, people underestimate how much weight is in the soil if the sides give out and just how dangerous that amount of weight moving can be.
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.
You can get the same endorphins with exercise, but you don't get to see the results of your work. It's so much more satisfying when you can clearly see your progress. Playing in sandboxes or digging holes in your yard is a game, but manual labor alone is often just work.
I haven’t been to a gym since leaving college (I rowed in a championship 8, and twice daily workouts were a thing). Instead I do manual labor and love it. I’ve built dry stack basalt walls, mowed, edged, pruned, chopped and sawn, poured and broken cement, etc and never missed the (for me) pointless repetition of “exercise”. I respect folks that can do it, but I can’t understand how.
Yeah, that's part of why those gameified exercise apps are such a big hit - you can see "results" before you see the results!