Minecraft wiki entry for Development_versions, associated text: Added new world/cave generation when upgrading saved chunks below Y=0 using the Caves and Cliffs experimental toggle.
Chunks previously upgraded with Bedrock underneath will not see this change.
Upgrades older flat worlds to the new extended heights available with the Caves & Cliffs experimental toggle
New flat worlds using the Caves & Cliffs experimental toggle will generate starting at "y=-64"