This is the famous oculus in the dome of the Pantheon in Rome. It looks sort of small in this picture, but it's actually 30 feet across. It helps maintain the structural integrity of the dome by removing a considerable amount of weight that would otherwise be at its most vulnerable point.
Annual rainfall exceeds that in London
It DOES rain inside the Pantheon. Gravity is still a thing. The floor is slightly convex to drain it away toward a number of well hidden drainage holes.
Hadn't looked it up, but did assume that design and construction included such drainage. Still, given that Rome has more heavy rains that drizzle, I would think that even rain, especially as it has become more acidic and polluted over the centuries, would be damaging to the floor.
As for other tourist attractions, I wouldn't care if storms washed away the Coliseum, especially after reading about recent activities there.
Very dramatic!