Those fancy robes are not just for show.
The teeth and fingerbones of holy men and more
The Pool of (add the name of the God or Goddess of your choice) is well known as one of the greatest holy places of the world. It is said to be full of piles of gold and jewels, a place where wishes are granted, wounds are healed, prophets have dreams that come true, and suspected criminals are fairly judged. Pilgrims from the other end of the world have been known to travel for more then a year, braving the perils of disease, brigands and bad weather, just to reach it's holy waters.
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?