I love to explore the game as well. I have no clue how people have already been able to reach the center, I keep getting distracted by awesome places to explore.
The arctic ones are currently my favorite. I'm exploring a pretty challenging one atm. Almost constant storms with near white out conditions, very few save points, long sprawling caves I got lost in for half an hour before finding a small opening in the ceiling. Thought I would warm up a bit in the water, first time I've seen a shark. There are lots of weird creatures around, a lot of them plants. I should call this one Little shop of horrors.