I started a new world at lunch so I could get reeds closer to my home base.
I've never been in a more cave-filled area. I dug my first house into a small hill near two cave entrances. I built a door, and as soon as night fell there sounded like 50 various creatures hissing and growling outside my cave door.
I started mining down, and hit a cave 7 blocks down full of more baddies. Went up the stairs, dug in another direction, another cave. Same in the third direction. Maybe I'm on top of a huge cave system, but all three were at different depths so I may be looking at several. Needless to say, my house sounds like night of the living dead with these horrible noises coming from all directions.
The sun came up, and I walked outside. Tons of enemies still there, chasing me around. Killed off a few, and noticed some zombies were still standing right above my front door. It turns out the new trees, when they grow big enough, can make a lot of shade. Enough shade for zombies to hang out in without catching on fire.
I killed off these zombies, dying once in the process. Now my big errand for the day is building a dirt ramp up to this huge tree and pruning the leaves so that zombies can't hang out under it during the day.