Are you tired of players simply digging down to bedrock and stripmining their ores, completely ignoring the fun and challenge of wandering lost and alone through hostile caves and dangerous ravines? If so, this mod will absolutely make your players love you!
This mod goes through world gen just before it finishes and if a stone block is not within three block lengths of a cave or air, it's replaced with solid bedrock. This means you have to use caves to find your ores and brave the dark depths. Remember to pack torches to light your way. Hint: place torches on the right wall when you're exploring, then when you want to get out, you can trace your route back by simply keeping the torches on your left.
This mod does come with a gameplay price though, underground bases are not as feasible and if you've got a hill near your house you don't like, you're stuck with it.
You can increase the amount of cave gen and ravine gen in the .cfg file, if you're not finding enough cave systems that go down far enough.
NOTE: Since worldgen is server side only, this mod only needs to be on the server, so client players can still happily connect and play if they do not have it.