Peter Richie's SimCity 4 project, called (what else) Megacity One, is like something out of a science-fiction story. Using 81 large city tiles, 26,542km of roads, 8,626km of subway lines and 324 hydrogen power plants, he has managed to create a single sprawling metropolis of 107,658,254 people.
For reference, the largest urban centre in the real world, Tokyo-Yokohama, has "only" 37 million residents. If you've ever been to Tokyo, and noticed how from many vantage points the city never visibly ends, imagine one three times larger.
Like many larger cities in the world that have spread so far they have overrun other big settlements, Megacity One actually comprises multiple city centres. The four here have things like airports placed in between, to minimise pollution and maximise density.
Some other fun facts: there are 512 water pumps, 486 waste management centres, over 2000 schools, 81 universities and 162 colleges.
The best fact, though, is that he did it all without the use of mods. It's 100% urban planning and SimCity 4.
If you want to read about the challenges of keeping the entire thing running, Richie's interview with Motherboard is good reading.