You don't die under flat world.

This is a bug in 0.11.0 and 0.11.1, if you dig straight down in a flat world, you don't die like normal, you just fall for a bit, and land on something. Basically, you can go under a flat world without dying. I didn't try this on a infinite or old world though, so I don't know if it happens on those as well or just a flat world.