Hello! I looked up this issue rather extensively and discovered that in 1.13, Minecraft changed the way it handled world generation in a significant way, which has a high tendency to break worlds that existed before 1.13. I'll start from the beginning and try to keep this short. The world is 1.10 because that's the last version a plugin called GiantCaves was supported. I am making a cave world and needed this plugin to create it. After creating the world, I used WorldBorder to load 10,000 x 10,000 of the world. I knew that by pre-rendering it, it should stop the world from generating default terrain where I didn't want it to. I then shut the server off, backed up the world (of course), then ran the server with Minecraft 1.14.4 (with GiantCaves removed). For much of the world, everything is still in tact. For other parts, however... Chunks are abruptly generated in the vanilla style. In this case, this is an ocean, so it places random chunks of ocean through the world. The terrain height you're looking at elsewise extends up to about Y210~, because it's meant to be a cave world. These holes did not exist when the world was in 1.10. I looked around and saw someone suggest --forceUpgrade. I attempted that with the world backup and, after a couple hours, ran the server again. Same problem in the same spots. The same chunks reverted to default. My question to you all now is... what can I do to fix this? Is there any particular reason why this is happening (in the same spots with the same chunks)? Would fixing these manually be an option, or would these chunks keep reverting back to 1.13 default?