From the record
“Saint Peter's Grotto, also known as Farmer South Dakota Grotto, is a Folk Art structure. Located in Farmer, South Dakota, it was listed on the National Register of Historic Places in 2001.”
Wikipedia article Saint Peter's Grotto, CC BY-SA 4.0.
One of 2 National Register listings published here for Hanson County · listed in 2001 · category: Structure.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service · the Wikipedia article