digdown

local function tryDig()
        while turtle.detect() do
                if turtle.dig() then
                        sleep(1)
                else
                        return false
                end
        end
        return true
end

local function tryDigUp()
        while turtle.detectUp() do
                if turtle.digUp() then
                        sleep(1)
                else
                        return false
                end
        end
        return true
end

local function tryDigDown()
        while turtle.detectDown() do
                if turtle.digDown() then
                        sleep(0.5)
                else
                        return false
                end
        end
        return true
end

local function refuel()
        local fuelLevel = turtle.getFuelLevel()
        if fuelLevel == "unlimited" or fuelLevel > 0 then
                return
        end

        local function tryRefuel()
                for n=1,16 do
                        if turtle.getItemCount(n) > 0 then
                                turtle.select(n)
                                if turtle.refuel(1) then
                                        turtle.select(1)
                                        return true
                                end
                        end
                end
                turtle.select(1)
                return false
        end

        if not tryRefuel() then
                print( "Add more fuel to continue." )
                while not tryRefuel() do
                        sleep(1)
                end
                print( "Resuming Tunnel." )
        end
end

local function tryUp()
        refuel()
        while not turtle.up() do
                if turtle.detectUp() then
                        if not tryDigUp() then
                                return false
                        end
                elseif turtle.attackUp() then
                        sleep( 0.5 )
                else
                        sleep( 0.5 )
                end
        end
        return true
end

local function tryDown()
        refuel()
        while not turtle.down() do
                if turtle.detectDown() then
                        if not tryDigDown() then
                                return false
                        end
                elseif turtle.attackDown() then
                        sleep( 0.5 )
                else
                        sleep( 0.5 )
                end
        end
        return true
end

local function tryForward()
        refuel()
        while not turtle.forward() do
                if turtle.detect() then
                        if not tryDig() then
                                return false
                        end
                elseif turtle.attack() then
                        sleep( 0.5 )
                else
                        sleep( 0.5 )
                end
        end
        return true
end

local function levelCycle()
  tryDigUp()
  tryUp()
  tryDigUp()
  tryUp()
  turtle.turnLeft()
  tryDig()
  turtle.turnRight()
  turtle.turnRight()
  tryDig()
  tryDown()
  tryDig()
  turtle.turnLeft()
  turtle.turnLeft()
  tryDig()
  tryDown()
  tryDig()
  turtle.turnRight()
  turtle.turnRight()
  tryDig()
  tryDown()
  tryDig()
  turtle.turnLeft()
  turtle.turnLeft()
  tryDig()
  turtle.turnRight()
  tryDig()
  tryForward()
end

-- Main code here

local tArgs = { ... }

if #tArgs < 1 or #tArgs > 1 then
  error("Usage: digdown <depth>")
end

depth = tonumber(tArgs[1])

while depth > 0 do
  levelCycle()
  depth = depth - 1
end