Go to the part with the poisonous gas maze. Get to the section aboveground that has two gopher holes on it. Near that area should be a breakable box with a Super Mushroom inside. Get the Super Mushroom and run to the two gopher holes. Walk over the holes, killing the alternating gophers back and forth until the mushroom wears off. It is possible to get 40 lives this way. Note - This works best with Wario.