The Grotto of Redemption
Believed to be the largest grotto in the world, Father Paul Dobberstein, a German immigrant, began building the shrine in 1912 after promising the Virgin Mary to build it if she healed him from an illness. Construction continued continuously for the next 42 years.
A grotto is either a natural or man-made cave and were common throughout Greek and Roman temples usually as shrines to various gods. The tradition was revived in Catholic churches across Europe, particularly venerating Mary. I saw a bunch of grottoes constructed in the eves of churches and palaces across Europe, none so large or elaborate as this one.
The value of the rocks and gems used to construct the grotto are estimated to be about $4,308,000.