16x16x16 blocks of landscape are generating over the top of existing landscape. They seem to begin at about level 50 or so and then protrude over the surface. They obliterate anything already there. So far there have been three of these, all in close proximity to each other and to my main base. I am afraid that I'll come back from exploring one day and find my house or a large chunk of my farm replaced with some random huge cube of landscape. What's going on and how can it be made to stop?

That was written yesterday. Last night it got worse. Now there are 16x16x16 cube voids appearing in the landscape. Quitting and reloading has no effect on either problem. There are three large cube holes in the landscape visible from the roof of my farm house. The sides are straight while the lower edge seems subject to terrain. They cut through any existing landscape, for instance caverns can be seen and one is full of water as there was a pool there before the cube intervened. In the screenshot 23.15.24 two holes can be seen, directly left and right of the reticle. The other screenshots show the contents of the three holes. Unfortunately I dug out the masses without thinking of taking a screenshot first.

NB This is NOT a chunk error. The masses and the holes are 16 blocks deep. There is a bottom to all of them. They are also all occurring at the same elevation. I don't want to merely repair these. I'd like to stop others from happening.