system data taking up 789 GB

My hard drive is suddenly filled with 879+GB of system data.

The only recent changes I'm aware of are a dropbox update. I use dropbox to store most large documents. I use selective sync.

I've read other threads on this forum and deleted a lot of old and duplicate files, but the problem seems to remain and worsen worsen. Each Time I check I have less free disk space.

Thanks for any advice.