Happend to me only 6 squars underground on a flatfield.
And two times the entrance collaps. Too the beginnig everything works fine. But after one or two hours of digging i can't use the entrance.
Was happy that i made Backups every hour so i could just got back to an working database.
So i'm keep on diggin deeper. A view hours later the entrance groundblock was half restored. But i can't remove it or do anything with it.
Lucky as i'm, i could still could use further on. Just had too spring offer that cliff when i go out off that tunnel.
Nearly at the same time the bug with the tunnel downward and the straight floor appears. Just when i hit the ironvein
If it's helpfull i could upload the backup files.
So i hope that the tunneltool is realy just an placeholder! If not would be a real hard punch on a nice game!
13.07. Looks like they have probably change something with the last update.
the second tunnel was recreated after the update and now everything is alright. Only the entrence is furcermore broken. so i digged a new one who is stable for now.
Hopefully there is more than that in the next updates.