Minecraft | Collapsing Block

Setup:
(Each of these is a Command Block, and must be built vertically)
  A
B   C    -> Negative Z
  D
A Command: /setblock ~ ~-1 ~ redstone_block
B Command: /execute @a ~ ~ ~ detect ~ ~-1 ~ stone 5 /summon FallingSand ~ ~-1 ~ {TileID:1,Data:5,Time:1,DropItem:false}
C Command: /execute @a ~ ~ ~ detect ~ ~-1 ~ stone 5 /setblock ~ ~-1 ~ air
D Command: /setblock ~ ~1 ~ stone

When all of these are in place with their proper commands, and facing the proper direction, place a Redstone Block inside to start it.