tunnel

-- fuel up!
turtle.refuel()

-- how far shall we go?
local requested_distance = ...
print(requested_distance)
requested_distance = tonumber(requested_distance)

-- how far have we gone?
local distance_traveled = 0

-- until we reach the goal
-- either move forward or
-- dig then move forward
while (distance_traveled < requested_distance) do
  if (turtle.detect()) then
    print("dig")
    turtle.dig()
  else
    print("forward")
    turtle.forward()
    distance_traveled = distance_traveled + 1
  end
end

-- reached the end so must turn around
turtle.turnLeft()
turtle.turnLeft()

-- now we need to move up one block
if turtle.detectUp() then
  turtle.digUp()
end
turtle.up()

-- now we've turned around and moved
-- up, ready to come back
while (distance_traveled > 0) do
  if (turtle.detect()) then
    turtle.dig()
  else
    turtle.forward()
    distance_traveled = distance_traveled - 1
  end
end

turtle.down()
turtle.turnLeft()
turtle.turnLeft()