It’s ambiguous.
When the Murrays bought out the Seversons’ interest in the land in 2005, the Severson brothers retained two thirds of the land’s underlying mineral rights.
- Two thirds of the mineral rights under all the land?
- All the mineral rights under two thirds of the land?
I confess I don’t know how they do these things; the sentence could go either way as far as the sentence structure goes.
What do you think?
Here’s a picture of the fossil that the article is about:

They mentioned that the photo is copyrighted, so I included the link again.