Sounds like you fell through the world and reached the bottom. With no fall damage these days, you were able to survive. If you look up you should see the city above you. In the past falling through the world meant you ended up in the death realm because once you hit, you died. When I fell through at the sewers entrance, I just kept going. My vertical position was at about -50,000 at one point. Luckily someone told me about double /unstick otherwise I\'d still be falling.
There are a few places you can fall through the world (last I checked) like the ramp on the main street just before you get to the windowless tower. There is a hole on the left hand side. There is also a hole behind the library big enough to fall through.
