[soneil]

> Also, if you have the computer checking everything, then those 5 people that are supposed to be redundantly computing the navigation plan are highly likely to be less diligent.

This is surprisingly easy to fix. If the computer notices before the human, call that a failure. Say, if the computer spots terrain higher than the current depth within X radius (that wasn't intentionally planned for), that's a failure.

I assume the military already has a regime in place to handle "you dun goofed". You can string failsafes after goofed but before the wall.