“ Whilst exploring underground the PCs discover tube like tunnels stretching for miles, upon further inspection the strange tunnels seem to connect all major cities in the are, what is their purpose?”
“ While it is within your purview to write long-spanning histories of your world, it is largely unnecessary. If you are running an RPG, do this instead: make a bullet point list of important events that affect the world today, and mysteries that people still don't know. Let these fuel your adventures.”