126.96.36.199 Beta, 188.8.131.52 Beta, 184.108.40.206 Beta, 220.127.116.11 Beta, 18.104.22.168 Beta, 22.214.171.124 Beta, 126.96.36.199 Beta, 1.18.0
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.