The same hole from another angle.

http://scienceblogs.com/aardvarchaeology/2010/06/more_subterranean_horror_in_gu.php

Large parts of Florida is sinkhole country. The harder limestone crust gets undermined as acidified groundwater eats away at the layers underneath. There have been horror stories of houses sinking out of sight but the sinkholes around here aren't usually so deep and, to anyone paying attention, there are usually clear signs that a sinkhole is about to open up.

Soft spots in the earth, concrete house slabs that are cracking and settling unevenly, Windows and doors that don't work right, surface water draining to cracks or holes in the earth, an odd hollow sound to the earth in a particular spot, the sound of dripping or flowing water are all clues. Funny thing is people often see the signs and live with them, sometimes for generations, but are surprised when the sinkhole opens up.

Around here it is usually during droughts, or after the first rains, when the sinkholes open up. As long as the water stays high the hollows are partially supported and stabilized. When the water level drops the caverns widen. If they get wide enough the hollow expands to the surface and the roof of the hollow falls in.