Silverfish
Silverfishes can infest stone anywhere in the Overworld.
Infested stone is not limited anymore to spawning under mountains.
It can be anywhere in the Overworld.
The configuration of the Silverfish can be found in .minecraft/config/sihywtcamd/arthropods.json.
Every Silverfish change is server-side. This means you don't need the mod on the client to enable these.
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infestedEverywhere: [true|false]to enable/disable silverfishes infesting stone anywhere in the overworld