Hi, I made some real progress - travelled 4000+ blocks away from home and mined diamonds, found obsidian, then practised laying a zombie trap with a small campfire down a hole.
Next time I logged in - it seemed I was dead, and my last spawn point and bed had been destroyed 'by fire'.
Now, I don't think the campfire was to blame.
I navigated back to the place I thought my mining base was - I'd laid a chain of mini-bases and monuments (and wrote down the coordinates).
When I got there - this is what I found. My house (circled) used to face the sea and a relatively flat forest behind it. Also, the nearby village seemed totally destroyed, with some houses split in half. Here's a photo or two...
The second picture shows the weird geometric larger grid squares that have gone missing or changed... water is sitting on the top of big blocks...
Your world was corrupted so the chunks got messed up and rearranged somehow, maybe due to a crash or similar.
If my experience is to be trusted it's quite likely that all the chunks are still around so if you can find the chunks that contain your chests you may well get your diamonds back.
Otherwise you can use Creative mode to give yourself what you think you've lost.
(Open the game to LAN and chose to allow cheats if you have cheats disabled. Don't forget to switch back to Survival before closing the game again.)
The only thing you can do is restore from a backup (try "restore previous versions" if you have not, but don't rely on this) - this is a game-breaking bug that has plagued the game since 1.8, becoming worse and worse over time - IMO, they shouldn't even consider releasing 1.16 until it is fixed (note that this also affects 1.15) - nor would I play on the latest versions due to this issue even if I already didn't (I've never experienced this in more than 6,000 hours of playtime over thousands of play sessions, and probably even more sessions on test worlds while developing mods, and all the crashes I've had while developing mods have never corrupted chunks like this, at worst resulting in an incompletely generated chunk if it happened during world generation - the file I/O system used by older versions appears to have been much more robust, possibly because the game wasn't as multithreaded, which sounds like a good thing but you have to be very careful that multiple threads don't modify the same data at the same time, or if they do, each thread is aware of the modifications):
MC-74762 Chunks Swapping (this mentions a full disk but that is not the cause in the majority of the cases reported; while this could cause chunk corruption in older versions it wasn't until around 1.8 that I saw people reporting this issue)
MC-161823 Chunk corruption in 1.15 & 1.16 Snapshots (currently with 155 duplicates and 5 related since last September, more than twice as many as either of the other two reports, with the first being much older)
If I encounter a seemingly unfixable issue when developing mods I don't just keep trying haphazard fixes - I revert to a last known good state and/or rewrite the code from scratch (often this means changes to vanilla code), either of which are usually far faster than trying to debug it - a bug like this would never make it to a final release (or any of the other serious bugs that often affect snapshots and pre-releases, for example, "Any worlds opened in 1.16 pre-release 1 are corrupt" - a simple test would have revealed this issue).
Hi, I made some real progress - travelled 4000+ blocks away from home and mined diamonds, found obsidian, then practised laying a zombie trap with a small campfire down a hole.
Next time I logged in - it seemed I was dead, and my last spawn point and bed had been destroyed 'by fire'.
Now, I don't think the campfire was to blame.
I navigated back to the place I thought my mining base was - I'd laid a chain of mini-bases and monuments (and wrote down the coordinates).
When I got there - this is what I found. My house (circled) used to face the sea and a relatively flat forest behind it. Also, the nearby village seemed totally destroyed, with some houses split in half. Here's a photo or two...
The second picture shows the weird geometric larger grid squares that have gone missing or changed... water is sitting on the top of big blocks...
One question: what happened?
And can I get my diamonds back?
Sorry that happened.
Your world was corrupted so the chunks got messed up and rearranged somehow, maybe due to a crash or similar.
If my experience is to be trusted it's quite likely that all the chunks are still around so if you can find the chunks that contain your chests you may well get your diamonds back.
Otherwise you can use Creative mode to give yourself what you think you've lost.
(Open the game to LAN and chose to allow cheats if you have cheats disabled. Don't forget to switch back to Survival before closing the game again.)
Just testing.
The only thing you can do is restore from a backup (try "restore previous versions" if you have not, but don't rely on this) - this is a game-breaking bug that has plagued the game since 1.8, becoming worse and worse over time - IMO, they shouldn't even consider releasing 1.16 until it is fixed (note that this also affects 1.15) - nor would I play on the latest versions due to this issue even if I already didn't (I've never experienced this in more than 6,000 hours of playtime over thousands of play sessions, and probably even more sessions on test worlds while developing mods, and all the crashes I've had while developing mods have never corrupted chunks like this, at worst resulting in an incompletely generated chunk if it happened during world generation - the file I/O system used by older versions appears to have been much more robust, possibly because the game wasn't as multithreaded, which sounds like a good thing but you have to be very careful that multiple threads don't modify the same data at the same time, or if they do, each thread is aware of the modifications):
MC-74762 Chunks Swapping (this mentions a full disk but that is not the cause in the majority of the cases reported; while this could cause chunk corruption in older versions it wasn't until around 1.8 that I saw people reporting this issue)
MC-150202 Chunks sometimes are dislocated / copied to another location
MC-161823 Chunk corruption in 1.15 & 1.16 Snapshots (currently with 155 duplicates and 5 related since last September, more than twice as many as either of the other two reports, with the first being much older)
If I encounter a seemingly unfixable issue when developing mods I don't just keep trying haphazard fixes - I revert to a last known good state and/or rewrite the code from scratch (often this means changes to vanilla code), either of which are usually far faster than trying to debug it - a bug like this would never make it to a final release (or any of the other serious bugs that often affect snapshots and pre-releases, for example, "Any worlds opened in 1.16 pre-release 1 are corrupt" - a simple test would have revealed this issue).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
Thank you both for your detailed replies! I imagined I'd accidentally blocked a lava flow and caused a build-up of volcanic pressure.
I'll gather details for a bug report...
BTW Minecraft version: Minecraft 1.15.2
Kind regards