start.lua

depth = 10
turtle.select(1)

for i = 1, depth do
 turtle.dig()
 turtle.forward()
 turtle.digUp()
 turtle.up()
 turtle.dig()

  if math.random() < 0.75 then
  turtle.turnLeft()
  turtle.dig()
  turtle.place()
  turtle.turnRight()
 end

 turtle.forward()
 turtle.digDown()
 turtle.down()
end