Nether portal spawned in a nether cave. Placed and ignited A BUNCH of TNT around the cave. Then I went back through the portal to the overworld. On the overworld side of the portal, there was a oversized (about 150% the size of a normal block), completely white, unmovable, indestructible block sitting there. i cannot break it, burn it, explode it, or interact with blocks through it. i can, however, walk through it.