So, I've been told that this is... "Buggy and caused by generating several worlds back-to-back."

It's basically seems to be half a cavern exposed to the surface. It's the labyrinth serpentine type of cavern. There's also a shaft roughly 30-40 wide that drops down 20 or so z levels to the cavern below it. The map is exposed down to level 95 straight off the embark level (140).

There is also a 2 square wide shaft that extends from the surface down to water in the 95th z level.

You might need to redo the embark. My default 4x3 gets a whopping 30fps. Also, come prepared, the natives aren't friendly.

This is the copied save before the embark. I tend to copy saves before I do an embark. So, although the screen shows the embark site as a reclaim that is not in this copy.

This is vanilla. No mods or wold gen tweaking beyond menu settings. Beasts set to High. World set to Savage. Covers 3 biomes.