The title says it all. It's a glitched city.
To do this, you need to be able to fly to Fuchsia City and Cinnabar Island. Also, you'll need a Pokemon who knows Fly and Surf.
1. Go to the Safari Zone. Pay the $500 and walk inside.
2. Go back and the guy will ask if you are leaving early. Say no.
3. Once you're back inside the park, save your game. Then turn it off.
4. Turn the game back on. Now walk out of the park again.
5. Here is where the first glitch appears. Instead of the guy asking if you want leave early, he gives you his welcome speech. When you asks if you want to join the hunt, say no. Now walk outside.
6. Fly to Cinnabar Island. Walk to east coast, and use Surf. Surf on the coastline. Just keep on surfing up and down that coastline for 500 steps.
7. After 500 steps, you'll see a message saying time is up. Exit the Safari Zone building. Your are now in Glitch City.
After walking awhile, you may get stuck. You can just use Fly to get out.