This is one of those unique places you really should visit at least once. Here’s what the website has to say:
The National Shrine Grotto of Lourdes is one of the oldest American replicas of the revered French shrine, dating to about two decades after the apparitions at Lourdes (1874), although the site had already been in use for more than seventy years as a place of prayer and devotion.
The Grotto was proclaimed a Public Oratory on December 8, 1965 by His Eminence Lawrence Cardinal Shehan, Archbishop of Baltimore. A place of pastoral beauty and spiritual inspiration, the Grotto draws hundreds of thousands of religious pilgrims and tourists annually.