Express Tools command FLATTEN
-or-
'flatten' using the "move up & back" method;
Command: move
Select objects: Specify opposite corner: xx found
Select objects: Specify base point or displacement: 0,0,1e99
Specify second point of displacement or <use first point as displacement>: <enter>
Command:
MOVE
Select objects: p
xxx found
Select objects: <enter>
Specify base point or displacement: 0,0,-1e99
Specify second point of displacement or <use first point as displacement>:<enter>