I ran into a frickin' crazy cavern today.
It had non-stop enemies. When I first discovered this, I had to take out 3 zombies, 4 skeletons, 2 spiders, and 3 creepers all at once.
The rest of the several hours of spelunking was pretty much the same, but me putting up constant walls to protect myself. I mean holy SHIT there were just non-stop creatures. I actually had to check to see if I had somehow put in on hard mode. Even though hard mode would have been easier than this.
I kept scouring for what was causing all this-- like 4 dungeons side by side or something, but nothing like that. I DID however find 3 different dungeons, but all very scattered, and with no possible access to the rest of the caverns.
I also noticed that creatures kept falling from above, so I theorized that they were spawning somewhere way up high, falling down here to get me, and then since they had "left" their area, it would respawn them, creating more and more mobs.
There was one spot where I could see several creatures of different types hopping around a gap, so I broke open the blocking piece (from a raised distance) and a FLOOD of zombies, creepers, skellies, and spiders. Easily 15 of them.
It was just non-stop though. I opened up into one area and literally say like 15-20 creatures crawling around it at once. Always coming, trying to get me, shoot me, etc. The skeletons were the worst.
I've done a lot of cavern crawling and no other cavern has been anything like this. I eventually used up half my diamond sword and about 150 arrows, along with 128 torches, and decided it was too much. So I finally put it on peaceful just so I could have a chance of making it out alive. I couldn't find my way out, so I decided to dig straight up and see where I ended up.
Where I started: An island not too far from my base
Where I went: wayyyyyyyyyyyy out west, then into the cavern
Where I surfaced: Right outside my main base
Awesome.
Here are some pics of the insanity-- I had to up the brightness so you could see. Another is a huddle of shit. Imagine that, non-stop (like, kill 3 creatures and 3 more are on the way).
This pic doesn't do the hoarde of enemies justice. You can look and see like 7 things here, but only because the other 15 crawled behind a wall.
Huge huddle.