I found this abandoned mineshaft floating above a lava ocean in the "Good luck" custom preset. Obviously most of it has burnt away, but you can still see what's left. In the gravel you can see that part of it is under the ocean. There are also a lot of cobwebs, presumably left from bits that have burnt away.
seed: 1214453399259054519
x co-ordinate: -430
y co-ordinate: 44
z co-ordinate: 220