It's interesting because, not too dissimilar to building tree houses, digging or tunneling is something kids are obsessed with that has a clear evolutionary advantage.

A nice tunnel is something you can use to store food in, stay safe from predators in, use for shelter or to stay cool in, have a shadow puppet theater.

I wonder if hes just got a bit of a positive feedback from some monkey brain circuits.

I think the GF should get him enrolled in some community college tunnel building classes or some such and demand he build it well and safely. Beyond that, it seems like a pretty benign hobby.