Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: As erosion by groundwater continues, the ceiling of a cave may collapse. The rock and soil above it sink into the ground. This forms a sinkhole on the surface. You can see an example of a sinkhole in Figure below . Some sinkholes are big enough to swallow vehicles and buildings. Answer: sinkhole
What is formed when the ceiling of an underground cave collapses?