Complex sentence: The context of building this grotto is the vision of Madona that an young girl experienced 18 times, and prior to that the Pope had also made the announcement of the dogma of the Immaculate Conception in 1854.
Split into simpler sentences: The context of building this grotto is the vision of the Madonna that a young girl experienced 18 times. Prior to that the Pope had promulgated the dogma of the Immaculate Conception in 1854.