| Clue | Source | Date |
|---|---|---|
| Coastal formation | LA Times Daily | 23 Dec 2023 |
| Hollow coastal formation | Universal | 23 Nov 2023 |
| Tidal carve-out | Wall Street Journal | 12 Aug 2023 |
| Neptune’s Grotto in Italy, e.g | Wall Street Journal | 29 Oct 2022 |
| Phenomenon carved by waves | LA Times Daily | 20 Nov 2021 |
| Italy’s Blue Grotto is a famous one | New York Times | 15 Nov 2021 |
| Partially submerged spelunking venue | The Washington Post Sunday | 18 Apr 2021 |
| Marine spelunker's interest |