Terraformed Mars

If not the most fascinating terrain on the Solar System, Mars boasts, at least, of the most diverse terrain that can be easily seen from Earth. Larger and closer than any land on the Outer Solar System and lacking an opaque veil like Venus, its differently colored and seasonally changing regions, clouds, sandstorms and polar caps are visible to our telescopes. Closer looks and the collection of topographic data have painted an even richer picture. The red planet has not only crater-strewn wastelands like the Moon and famed volcanoes but also terrain clearly molded by large volumes of fluid in the distant past. Many fertile imaginations have projected biospheres over this terrain, among them Chris Wayan, who made a beautiful portrait of the climate, biomes and life of a Mars with a proper atmosphere and a lot more water.
This presents an unique opportunity. A massive heightmap combining data from NASA’s Mars Global Surveyor (MGS) and ESA's Mars Express (MEX) is available at the USGS Astrogeology site. After shrinking it I took a large chunk of the map. It spans the entire Valles Marineris in longitude and in latitude goes from the Argyre to part of Tempe and thus has some of the best real estate on the planet. No Olympus or Hellas but it's a good sample. I got this smaller heightmap through WorldPainter. Now I had a "raw" map entirely made of red sand.
Using Wayan's Mars Reborn (itself inspired by Kim Stanley Robinson) as a reference I grafted oceans, rivers, lakes and vegetation onto the map. The rivers were the hardest to make. Only small parts of them are flooded land. Most are layers of air and water blocks replacing terrain. They do not stay at sea level but rise with the terrain. Some rise on and on until they're ice blocks in the mountains. This took forever to make and then once testing I had to correct errors on nearly every single stream in the map. There were three points on the map which had some work done by hand on Minecraft and not WorldPainter.
The vegetation follows an overall logic, going from jungle to spruce according to latitude and altitude and thinning from the coast to the planet's interior. It is not, however, the most realistic representation. A faithful representation of what terraformed Mars would look like is a lower priority than playability, variety and coolness. This extends to some of the stone and mesa types used in the surface -there's a corner of the map with all kinds of terracotta-, and to the coal present underground which most certainly is not in real Mars.
Survival compatibility is almost full: there are ores underground, a village, stronghold and most items.

If you're using Optifine or a similar mod I recommend raising cloud height. Extending view distance will make for a prettier landscape, as in any custom map.

For this I have to thank two friends who made the village and stronghold, WorldPainter, NASA, ESA, Chris Wayan and Kim Stanley Robinson.