I visited a horizontal mine tunnel which I had dug out several days ago. Instead of being able to continue through the tunnel I smacked into a semi-transparent black space (screenshots attached). It appeared to be a rectangular column extending forever both up and down. I could see across it to where my mine tunnel began again, but could not walk through the space. I could not place blocks in the space, dig with my pickaxe in the space or place items in the space.

I dug out a side tunnel to go around the space, and was able to reach the other side, as well as dig up next to the space and hit its "edge" with the same results as earlier. I sometimes took damage when getting too close to the edge.

The last thing I did in this vicinity was place some rail down one side of my tunnel, which ended about 12 blocks before the black space appeared. I did run a minecart off the end of the rail with no stopping mechanism, but it stopped one block from the rail as usual.

I have been playing this world in Peaceful, occasionally switching to Easy. I was in Peaceful immediately before and after I discovered this bug.

Images:
1. Looking from my tunnel across the black space, you can see where my tunnel starts again
2. I dug around the corner of the black space to see if it behaved the same way all around...it did.
3.A view from ground level
4. Ground level view showing how it cuts up through trees, and presumably, through the empty air space above.