Definition of "grotto"

...But, while we are waiting for the temple, can you not show us the altar?—we want to see your grotto."Pope desired nothing better than to show his new toy, and led the way to the pretty and fanciful cave, which was but just finished.

1837, L[etitia] E[lizabeth] L[andon], “Pope’s Villa”, in Ethel Churchill: Or, The Two Brides. […], volume I, London: Henry Colburn, […], page 192

An earlier attempt to organize a grotto in the Indiana, PA, area in the mid-1970s failed to succeed, but from it developed the informal Chestnut Ridge Explorers Association.

1987, National Speleological Society, NSS News, volumes 45-46, page 331

By the mid-1940s members of local grottos (regional clubs of cavers within the National Speleological Society) were exploring Porcupine Cave (Bloch, 1946).

2004, Anthony D. Barnosky, Biodiversity Response to Climate Change in the Middle Pleistocene: The Porcupine Cave Fauna from Colorado

The answer to the question, “Are there any caves in Arizona?” was always the laconic “None to speak of,” with emphasis on the “to speak of.” This secrecy dovetailed with the cave conservation ethic promoted by the local caving grotto.

2008, Neil Miller, Kartchner Caverns: How Two Cavers Discovered and Saved One of the Wonders of the Natural World