Mining question

So I found one of those big chunk of black walls. Usually i dig in a straight line, put a fire, come back later, put another fire, etc
Eventually the entrance to the lower level will spawn.
This time, i decided to dig all around the back walls, and lit fires everywhere, to save some time.

I seems the entrance will not be here this time. ^^
So how does it work? Each "black" tile you mine has a chance to spawn the entrance? Is it a specific tile?
Does it still work if there is no rock tiles left all around the black tile?
What is your technique if you feel like sharing.
Can we spawn several lower level entrance? Or once one is spawn, all black walls disappear?
Eventually the entrance to the lower level will spawn.
This time, i decided to dig all around the back walls, and lit fires everywhere, to save some time.
I seems the entrance will not be here this time. ^^
So how does it work? Each "black" tile you mine has a chance to spawn the entrance? Is it a specific tile?
Does it still work if there is no rock tiles left all around the black tile?
What is your technique if you feel like sharing.
Can we spawn several lower level entrance? Or once one is spawn, all black walls disappear?