Do not open a 1.5.2 world in the latest version (1.20+). The biome generation, world height, and block IDs have changed so drastically that it will corrupt the landscape, spawning floating gravel blocks and "Unknown Block" errors. Always play 1.5.2 worlds in version 1.5.2.
The Ultimate Guide to Minecraft 1.5.2: World Downloads, Seeds, and Nostalgia
Do not open a 1.5.2 world in the latest version (1.20+). The biome generation, world height, and block IDs have changed so drastically that it will corrupt the landscape, spawning floating gravel blocks and "Unknown Block" errors. Always play 1.5.2 worlds in version 1.5.2.
The Ultimate Guide to Minecraft 1.5.2: World Downloads, Seeds, and Nostalgia