Fictional Sinkholes

1. Grimpen Mire, The Hound of the Baskervilles, Sir Arthur Conan Doyle
2. Peter’s Pot, Clouds of Witness, Dorothy L. Sayers
3. The Gulper, Gone-Away Lake, Elizabeth Enright
4. The snow sand (lightning sand, in the movie) of the Fire Swamp, The Princess Bride, S. Morgenstern (trans. William Goldman)
5. Blackberry Bog, Lassie and the Mystery at Blackberry Bog, Dorothea J. Snow

Any others?