Subterranean trees

The history of plants is also underground, and not just in the roots:

http://www.philly.com/philly/blogs/real-time/Wooden-water-mains-Spruce-Street-May-2017.html

A very much older tree was also found underground not far from there, a number of decades ago:

http://www.biodiversitylibrary.org/item/112143#page/142/mode/1up

To read more about that, see here (Chapter 1): http://fitlersquarepress.com/chapters/