How limestone sinkholes are formed
3 February 2014 Last updated at 01:20 GMT
In March 2013, a Florida man, Jeffrey Bush, died after a huge sinkhole opened up under his house.
Florida is the sinkhole capital of the world: Most of its landmass is a vast limestone platform, flushed by groundwater below and rained on from above.
Acidic rainwater eats away at the limestone bedrock, forming cavities in the subsurface. Sometimes, collapses can drag down whatever's on the surface into a subterranean world that lies beneath.
For a BBC Horizon documentary on sinkholes, Prof Iain Stewart saw how sinkholes can begin to form in limestone rock.