Question

What was Benjamin Franklin's idea about Earth's interior?

Answer 1

A very dense liquid state of matter.

Explanation:

Benjamin Franklin described an Earth with thick, dense fluid inside which causes the surface crust to be violently broken and mobile – no one else would come closer to describing the modern theory of plate tectonics for almost two hundred years.
https://mountainmystery.com/2015/01/17/ben-franklin-geophysicist/