Cave ins happen. They can happen even on surface walls on your first attempt at mining. There are supports to stop them but there is no indicator of the supports range.
Although at the moment cave ins aren't fatal, just common as hell. I must have survives 10+ of them by now.
I'm not surprised that there's bog iron in this game, seeing as they probably wanted at least a low-quality metal to be available to newbies and there not to be such a divide between metal players and non-metal players.
And yes, bog iron is a thing. I have tons of it at my real life place and it's everywhere.