"This City of Thunder is too difficult to subdue. Your Primordial Spirit has been refined well; quickly transfer it back into your body and come to help us together with Daoist Equal to Heaven!" Enlightenment Dao, using his Divine Skills continuously, tried to strike the city with Divine Thunder but failed to breach the defenses, naturally leaving him unable to refine the City of Thunder.
The Ancient Thunder Cannon needed thunder to supply energy, but this place was originally a Sea of Thunder where thunder was endless. Using a bit of it would cause more to regenerate. Although the Ancient Thunder Cannon greatly consumed the thunder, in just a moment of thought, the thunder lost from the Sea of Thunder would be replenished.
A single Ancient Thunder Cannon seemed to use as much water as a river, but how many rivers can an ocean accommodate? Endlessly many!
