In all seriousness, digging tunnels is a military tactic that gets some use sometimes. The Russians have done it in the southern part of Avdiivka. I haven't heard of a case where militaries used an actual tunnel boring machine, though.

Tunnel boring machines are not just extremely expensive, they're also extremely bulky and highly visible when on top of the ground. If you ship one to where you want to start your tunnel, there is a high risk that enemy intel will spot it, ruining the surprise.

The actual digging is extremely slow, meaning there is no way of knowing whether the tunnel will even still have some tactical use when you're done.

Once you emerge on the other side, it won't be long until the enemy notices your exit and does something about it, like bombarding it, stationing troops there or sending something of their own back through the tunnel. That means even if you succeed, you only have a very short window to do something with your tactical advantage, and said something will very likely be a suicide mission.

Tunneling through the loose mud of Ukraine would not be easy. You can do it if you send a work crew after the machine to immediately build strong walls behind it, but that would be even slower and more expensive.

All in all, it would just be more useful to use either much smaller and shorter tunnels that don't use tunnel boring machines or utilize some form of artillery or drones to achieve the desired effect behind enemy lines.

A superlative suggestion, sir, with just two minor flaws. One, we don't have any tunnel boring machines, and two, we don't have any tunnel boring machines. Now I realise that, technically speaking, that's only one flaw but I thought it was such a big one it was worth mentioning twice.

Y'all questioned it at the time, but this is precisely why we signed a mutual defense treaty with the molemen.

It wouldn't work because it's a boring strategy.

I bet it could be done if it was purpose built and not a big old slow one that was designed for huge car and train traffic sized tunnels.

Make it smaller for man sized tunnels, it will be much faster and cheaper. Basically drone-ified. Could really catch the Russians off guard.

It would also help reduce the chances of it being detected above ground while it's being moved to the starting point and bleow ground when it's digging.

Russian air defense struggles as it is I doubt they'd be fully prepared to monitor for tunnel activity.

Battle of five armies in Ukraine when?

Because it's cheaper to nuke Russia than use many boring machine

The war would be over by the time they make it... assuming the other side never catches wind about it.

It's definitely possible, I remember there was a documentary about it

Cost.