Tectonic is a large scale worldgen datapack (packaged as a mod for Forge, Fabric, and Quilt) that significantly alters overworld generation. There's mountain ranges far larger than those you would find in vanilla, underground rivers that can go on for thousands of blocks, and more interesting terrain generation for you to go out and find on your adventures with this pack.

This pack also improves on vegetation as well, with vastly improved oak and birch trees (with more overhauled trees coming soon!), improved river generation, and other smaller terrain changes. On top of this, there are over 25 new biomes and biome variants for you to explore. All of this is done without adding a single new block or item.