Florida is a giant sand bar. You need not dig very deep before hitting water. This presents problems with construction as roads and buildings make surfaces impervious to rain, causing a flooding hazard from runoff. Retention ponds are dug to mitigate flooding from development. While natural lakes in Florida are shallow, retention ponds are deep, penetrating the ground 40 or 50 feet. But, because of the high water table, these ponds need to be pumped while under construction. In the woods to the left of the image is a temporary holding pond. Click on the image for a larger view.