Top floor press:
d -> j (downward stairway) and mine a stairwell
floor below:
d -> i (up down stairway) directly below
If needs be, d -> d to dig out a tunnel connecting your new stairs to the old area.
EDIT:
This is what he has done. He's dug out the surface for some reason and there were underground pools just beneath the surface. This is why it has all flooded.
