In the land of Aviontix there are ruins. A lot of ruins. There are many sand buried cities, king’s tombs (They were a very religious people, focused upon their leaving to the afterlife), and huge statuary. However, Aviontix was a kingdom so long ago, that everything has been looted. (Figure there has been a whole historical epoch or two since then) There might be a hidden tomb or something somewhere in the lands, but it is unlikely. This does not stop people from selling maps to them all the time.
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?