Dig a tunnel, 4 blocks wide, down to bedrock

for t=1,100 do
    turtle.digDown()
    turtle.down()
    for d=1,4 do
        turtle.turnLeft()
        for x=1,3 do
            turtle.dig()
            turtle.forward()
        end
    end
    for i=1,9 do
        turtle.drop()
    end
end