What's interesting about this to me is the way it sort of causes you to examine why certain things are acceptable hobbies and other things aren't. This particular hobby has three notable attributes:

1. It's risky.

2. The end product doesn't seem particularly useful if we set aside the mental/physical benefits. i.e.: it's done for it's own sake.

3. It's novel.

I would argue that 1 and 2 are attributes that a great number of hobbies share. Skiing, Mountain biking, football, kayaking, woodworking, sitting in a chair and playing video games all weekend. All these things carry some very specific and often non trivial risks to health or bodily harm over time.

Of course because these hobbies aren't novel, the benefits of the hobby are well understood. Our understanding of the risks and how to mitigate them are also quite a bit more understood.

It seems to me that the real 'problem' with this tunnel digging hobby is how novel it is. Which consequently leads to a lack of knowledge about what the risks and benefits may be.

Of course it's possible the poor guy is just dealing with a mental illness.