Tectonic is a world generation datapack and mod that aims to transform how terrain is shaped.
The way mountains generate has been completely overhauled, with them forming huge mountain ranges that can stretch for thousands of blocks. These stretch higher than vanilla mountains do, reaching peaks above y300!
In mountainous terrain that is too towering to allow regular river generation, terrain gets carved for rivers to continue underground. These link right up to the regular rivers, so there's no interruption in exploring along rivers.
One of (if not the most) notable changes to generation you'll see with Tectonic is that terrain is much smoother than normal. This means the strange staircasing on some hills - also referred to as "lerp" issues - is gone.
Oceans are much deeper than they are in vanilla, with deep oceans even extending into the deepslate layer!