mine

function fuel()
  if turtle.getFuelLevel() <= 50 then
    turtle.select(16)
    turtle.refuel(1)
  end
end

function isValuable()
  local isJunk = false
  for i=1,4 do
    turtle.select(i)
    if turtle.detect() or turtle.compare() then
      isJunk = true
    end
  end
  if isJunk then
    return false
  else
    return true
  end
end
function checkWalls()
  for j=1,4 do
    if isValuable() then
      turtle.dig()
    end
    turtle.turnRight()
  end
end

-- ( Program Start ) --

local depth = 0

fuel()
turtle.digDown()

while turtle.detectDown() == false do
  fuel()
  turtle.down()
  depth = depth + 1
  checkWalls()
  turtle.digDown()
end

for k = 1,depth do
  fuel()
  turtle.up()
end