Spawn in a huge swamp. Everywhere that I've explored, the swamp area, has huge underground caverns. The all have lots of resources. Skeleton spawner at x: -244.44227 y:35.000 z:89.54670. Spider spawners at x: -285.12468 y: 39.000 z: 75.45409 and x: -283.49035 y: 25.000 y:457.30000. The second spider spawner is located right off from an abandoned mineshaft. The second spider spawner also has another spider spawner right next to it. Zombie spawner at x: -334.48870 y: 40.000 z: 485.81613. The zombie spawner is in the same cave system that the second spider spawner is in.
X: -25 / Y: 63 / Z: 23
X: -72 / Y: 36 / Z: 19
Huge lava caverns. Lots of resources. Lots of chutes off the sides. Easy to find.
X: -214 / Y: 66 / Z: 118
Entrance to a cave with two spawners in it. Very dangerous!
X: -217 / Y: 64 / Z: 445
Entrance to large cave system with abandoned mineshaft and spider spawners and zombie spawner.