Structures can replace bedrock on top of the roof in the "Caves" world type.

Overview:

As we can see in the pictures, ponds and Netherrack from the ruined portal can spawn on top of the "Caves" world type.

My worlds:

1. "Lush Caves" Buffet caves generation: "7155732897881131303" /tp @p -287 320 -300
2. "Warm Ocean" Buffet caves generation: "-5672041926625585878" /tp @p 132 320 55

Steps to reproduce:

1. Create New World
2. Game Mode: Creative
3. Select the "Caves" world type in "More world Options...", click "Customize" and in the "Buffet world customization" panel select "Lush Caves".
4. Write a seed: 7155732897881131303 in the "Seed" text field.
5. Click "Create New World"
6. Teleport: /tp @p -287 320 -300
7. Enjoy swimming in a bedrock pond!