It may be days if not weeks before structural engineers come to a definitive conclusion about what caused a high-rise condo to collapse in Surfside, Florida early Thursday. But one educated theory comes from a professor at Florida International University. Shimon Wdowinski studies the process of subsidence – the sinking of land usually by man-made causes, such as mining, drilling for gas and oil, or more commonly, the extraction of subsurface water. His 1990’s study confirmed subsistence was occurring at the Champlain Towers condo 30 years ago. Doug McKelway has more.