Make the build hight 1000 all the way to -400

The height limit should be 1000 and the bottom height limit see should be -400!

This would be very useful for mining down to the depths of the deep, deep dark.

And you could build the biggest structure to the build limit. You can't build that much when most mountains are at y80, and the limit is 300. You could build huge railway tracks all the way up on servers, and make the clouds higher, too. And you could build a true house in the sky!