Unearthed HFS at Elevation... 6...

I was digging down and opened up this construction at Elevation 6, which is above the first cavern layer. This rectangular section has caverns, but all this space just leads down to slade not too further down.

When I started this fort about an hour and a half ago my FPS was at 0 for a while despite no waterfall to fill, followed by alerts that there were cave ins below.

As soon as I uncovered it a Forgotten Beast flew up and killed 25% of my population, lol.

Anyone else had an embark like this?

https://preview.redd.it/efdeboi9eb3b1.png?width=1321&format=png&auto=webp&v=enabled&s=1671945fa785070cbcebe971d8466ef8bae847cc