On Java Edition, the fossils, which is the rarest structure you can stumble upon while inside a cave below a desert/swamp, don't generate below y=0. Not only that, if it does, it doesn't generate with deepslate diamond ore replacing the coal on it.
Step to Reproduce
1. Find a swamp/desert biome. You can use this seed to strip out the stone to find fossils under a desert biome. -393253037
2. Clear out the stone variants/deepslate/gravel/dirt using command blocks with the following command put into repeating command block.
/execute @p ~~~ fill 10~10~10 ~-10-20~-10 air 0 replace deepslate
/execute @p ~~~ fill 10~10~10 ~-10-20~-10 air 0 replace tuff
You can find fossils below y=0 with diamonds replacing coals.
No fossils generating below y=0 at all.