So I just logged on to my survival single player world, thinking to find a few more diamonds. Previous to saving and quitting previously, I had been in my mine, well, mining. So I just log on, and whaddya know, I'm encased in stone!! I was pretty freaked, so I saved and quitted and got back on to it, and I was still in the ground! So I use my efficiency 4 pickaxe to stop suffocating, because I was, and I was about 5 blocks directly below one of my tunnels at level 11. I was just above bedrock prior to escaping. (There were also diamonds right next to me, although it isn't relevant) To make this clear, this is the very very very first time this has ever happened to me, and I've probably saved and quitted in this world alone hundreds of times.
What I expected to happen was...: Upon returning to my world, I would be right back in my air-filled tunnel as usual.
What actually happened was...: I spawned encased in stone, right above bedrock
Steps to Reproduce: I have honestly no idea, it seems to be totally random.