About 5 years ago, I set myself to finish Pixal. One of the big things I had to do was creating a huge world which players could explore with their expeditions.
I begun by pasting together the output of several runs of a fractal/voronoi terrain generator, doing small modifications for them to fit.
In order to ease the edition of the world, I divided it into 350 32×32 tiles sectors (total world size: 800×448 tiles). My sister was entrusted with the huge work of creating the individuals maps by hand.
In addition to the ground level, we also created a subterranean / underwater map and another one for the “skies”.
There was a server side component which swallowed the TMX maps into the database and allowed players to browse the world. Then, to make scale even grander, I allowed zooming into each one of the tiles, generating another persistent map (32×32 I think), in which you could create your own buildings, or explore caves (randomly generated using CA). Then the project collapsed under its own weight, even though all this hard mapping work was finished.
In the end, Pixal was never released, and this world was left unexplored.