Caveman2Cosmos is a CivFanatics community mod that add new features and content to the Civilization 4 experience, with almost daily updates since November 2010. It is the spiritual successor the The Civilization 4 mod Rise of Mankind and its addon A New Dawn, and combined the work of over 100+ modders to create a mod to "rule them all". It is developed by StrategyOnly Productions with its modding team made up of prominent members of CivFanatics.
C2C is the largest BTS mod I have ever played, the amount of added content is just immense. On the quickest game length you're still looking at a minimum of 100 hours put in, and you can easily go beyond 100 hours of playtime.
There's quite a bit more to be said about it, but, having played many many mods since I started playing Civ 4 some 8 years ago, C2C is absolutely in my top 3, and has managed to keep me invested in a playthrough far longer than any past mod or scenario I've tried. I absolutely recommend playing it if you're a fan of the civilization series.