This 3D model is based on the Sun Stone, a monolith/sculpture that was uncovered in Mexico City (The Aztec Capital) in the late 1700s. It's exact meaning is debated - it could be religious, cosmology related, geographically related, a calendar, etc. I just think it looks cool!
My approach was to sketch it out, and extrude different sections to different heights. I would have loved to sketch it in two heights so that you could paint the calendar, then wipe off the high spots, but with such a complex model I decided that would leave the model looking too simple.
If there's any demand, I can try to cut the model in two and see what he result is.