Could it be possible that the minecraft plains were hit by a meteor?
Here is a crater leading to small network of caves, ravine and mineshaft with lots of ores and lava to keep you entertained.
I found one of these on a world. Except it was like 30x40x20. It was deep enough to kill me when I was wearing armor that (at the time) gave 20 protection. I was full-hp.
Very easy to find diamonds in this seed.
I fell into one of those wearing full armour and had full hp. But the cool thing is, is that I survived. Had 2 hearts left. It was really deep too. My heart was racing while I was falling thinking I was going to die.
This is one of my favorite seeds ever. Sadly, after I had done very well and found diamonds, because I had used a snapshot everything in my chests got DELETED and I had to start all over again.