swallow hole 
noun
Another term for sinkhole.The borough surveyor diagnosed it as a swallow hole, which occurs when large deposits of sand trapped in the chalk are washed away during rainy weather, leaving a large void which then collapses....
- She was walking towards us and all of a sudden, she just went down to her shoulders in what we'd call a swallow hole.
- Its precise location provides intriguing possibilities for a natural feature - tree, swallow hole, spring - to have been its original focus.