I flew around in creative mode and tunneled down into an abandoned mineshaft! It wasn't that exciting though :/
So I decided to stand near the spawners and spawn as many spiders as I could.
Here you can see two cave spider spawners. One is in the upper right corner, the other in the lower left.
I got an aerial screenshot showing how many spiders there were.
I got too close and they started eating my face off.
I also found some really good stuff in there, like lapis, gold, iron, coal, and melon seeds! I do think they need some work, however. Like not spawning in caves. Or ravines. Or in any sort of open air.
EDIT: Moments after posting this, I found an enderman!
God, these things are scarier than I thought. I looked at one, and he didn't immediately teleport when I looked away. I thought it may have bugged or something. But then, as I walk away, he teleported right behind me. I didn't get a screenshot, unfortunately, since I was in oh god kill it mode. He did drop an Ender Pearl though.
I couldn't make it work on an enderman though. :/
Also, a screenshot of the loot I found in the chests:
Everything in the chest is what I found. The bones were from a skeleton, not from a chest, as awesome as the would be. The loot is nice, and I'll have to go plant those melon seeds right now. I'll update on the melons later.