What does the cannon that launches every day at nine?

At 9 P.M., every last eventide, a convention that started in the eighteenth century-the Firing of the 9 O'clock Cannon. The 9 o'clock gun is discharged from the antediluvian Fortress of San Carlos de Cabana. Primitively, two shots from the gun were finished. The rudimentary shot was deliberate to prognosticate when the gates of the city would be open, with the 9 o'clock gun gesticulates when the city's entryways would yet again, be shut.

Ithink its b) built underground reservoirs