What I do is I mine a 3x1 tunnel straight down to about level 9 or so while putting ladders the whole way. Then I dig a boss room 21x21x4. Then I put a 3x1 tunnel every 2 blocks so its like TSSTSSTSST (T-TUNNEL S-SPACE). Then I put a terd load of chests for all the diamonds ima mine. Then I mine lika boss to find diamonds. Or if u want boring coal and iron, mine like an assistant to regional manager. This strategy is beast. One time i asked my mom to pick a number between 1 and 28 (cuz theres 28 tunnels) she picked 9 and i started mining. I found 15 diamonds in under 10 minutes. That 1.5 diamonds every minute, no joke. So ya dis iz beast.