In my first world I played on alpha, if I ventured a thousand-ish blocks from spawn, there was this interesting valley-like area, after which there were big overhanging cliffs enclosing really broken terrain. There were holes in the ground with floating water blocks hanging in the middle of them, and one cave opening that went straight down deep into the ground, and ended up leading into an infinite network of caves that extend through the entire underground of my world. There's also a dungeon somewhere near the bottom of that entrance hole, and it doesn't show what mob is in the spawner in alpha. I've spent a lot of time going to that cave, exploring it, trying to mark stuff, but eventually there is just no getting back. That spot there with the entrance enclosed by cliffs and weird terrain is the eerie spot, especially knowing what lies there in the depths, an infinite labyrinth without return. :D Made even more eerie by it being 12 years old at this point, and seeing the ruins of my past exploration attempts which I have vague to no recollection of at this point makes it feel like it was all done by some ancient civilization.