I started building a treefarm, digging out a 29x21x8 room for my future wood supply. Obviously, lighting was needed in the ceiling, so I thought I'd place glowstone up there. Then, as I started placing glowstone, I thought, "why not just have the whole ceiling glowstone?" So I did.
Then Jiggy came along, and suggested that the walls should be glowstone too, and started making it so.
So, we had a big room with a glowstone ceiling and and glowstone walls, what now? Glowstone floor! Sod the trees!