Why did I find underground Clay?
I was mining for iron when I found clay, not exposed to water, at Y=16. How could this happen? I’ve never seen it until now. I was mining under a roofed forest near an ocean.
After 1.18 update:
Clay can generate in Lush Caves biomes in the form of blobs. Clay attempts to generate in blobs of size 0-160, at any height.
So it's definitely possibile you found it at Y=16, and you problably are near a Lush Cave.