I dont know what this is, figure it out for yourself. Basically create a new world, select ‘old’ as your map type, turn on caves and cliffs experimental thingy. Load in the world then leave it. Make infinite. Rejoin. Enjoy your sunken chunk of land, your easy access to ores and caves- plus its all in survival so you can get achievements way faster.
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Thank you for your report!
We're tracking this issue in MCPE-122042, so this ticket is being resolved and linked as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature to see if the issue has already been mentioned.
Quick Links:
📓 Bug Tracker Guidelines – 💬 Community Support – 📧 Mojang Support
📓 Project Summary – ✍️ Feedback and Suggestions – 📖 Game Wiki