droptunnel

local args = {...}

if args[1] == nil then
  print ("Specify the depth to tunnel and place ladders in the turtle's inventory slots, starting at 1.")
end

function step()
  while not turtle.forward() do
    turtle.dig()
  end
end

function stepBack()
  if not turtle.back() then
    turtle.turnRight()
    turtle.turnRight()
    step()
    turtle.turnRight()
    turtle.turnRight()
  end
end

function stepUp()
  while not turtle.up() do
    turtle.digUp()
  end
end

function stepDown()
  while not turtle.down() do
    turtle.digDown()
  end
end

local depth = tonumber(args[1])
local slot = 0
local ladders = 0

for i = 0, depth, 1 do
  if ladders < 1 then
    slot = slot + 1
    ladders = turtle.getItemCount(slot)
    turtle.select(slot)
  end
  stepDown()
  turtle.dig()
  turtle.place()
  ladders = ladders - 1
end