Loading world in 1.16 destroys existing structures

I've just updated from 1.15.2 to 1.16. Upon loading my existing world into 1.16, I spawn in a dirt block, and when I respawn, almost all of the "base" that I have built has been taken over by a mountain and the terrain around it changed. All the terrain also seems to have become strangely structured and and almost like "pillars".