Hi again.
I figured out how to use to Minecraft clone command.
It appears that these aren't ordinary sinkholes.
The terrain seems to have been copied from some other location,
since when I raise the elevation, the top of the land masses do not match up with their surroundings.
Thanks very much,
Richard

Hi there.
That doesn't address the 'Mem: 35%' line in the screenshot.
Doesn't that mean that minecraft has enough memory ?
And, if so, that also doesn't explain the sinkholes that occured after upgrading to 1.8.4.
So the question still stands - where did the sinkholes come from.
Thanks very much,
Richard
ps. - and, no, it wasn't my little brother - lol

Hi there.
In the comment from Kumasasa:
"You've run out of memory, see top left corner ("Allocated: 100% 3993MB")."
That is the 'allocated' memory, not the used memory.
Minecraft seems to be using only 35% of the allocated memory.
From the line above the 'Allocated: 100%' line:
"Mem: 35% 1421/3993MB"
It says Allocated 100% because of my command line parameters to Java:
-Xms4G -Xmx4G (starting allocation, maximum allocation)
I found another, more severe sinkhole, also in my camp. see image.
This one is also one chunk, but sunk much lower.
Also note, in the top =right= corner:
'Mem: 69% 2760/3993MB'
Thanks very much,
Richard

I have an area of land near my camp that simply dropped in elevation.
The area is enclosed in one chunk.
The terrain appears to have been lowered by 7-8 blocks.
A short while after noticing this, I also saw that I lost 4 Stone Pickaxes, and one Diamond Pickaxe, from a chest.
I don't know how to reproduce this bug.
I wish I could tell you more, but this is all I know right now.
Thanks very much,
RIchard