A Grotto

1. A small cave.Grotto - Wikipedia
2. An artificial cavern-like retreat.
3. A Marian shrine, usually built in a cavern-like structure.
4. A local organization of cavers that typically organizes trips to caves and provides information and training for caving; a caving club.
5. (Satanism) A secretive name for a local group of underground Satanists.