Read the following selection and answer the question. On a September day in 1999, people began to notice that Lake Jackson in the Panhandle region of northern Florida was shrinking. Within a few days, it was almost gone. A sinkhole had opened beneath the lake and drained it, along with all of its inhabitants including fish and alligators. As aquifers lose water, rock and soil layers can become weaker and less capable of supporting overlying layers of earth and any human structures built upon them. In such cases, the land surface above may subside, or sink. Sometimes subsidence can occur locally and suddenly in the form of sinkholes, which are areas where the ground gives way with little warning. Once the ground subsides, soil becomes compacted, losing the porosity that enabled it to hold water. Recharging a depleted aquifer may therefore become more difficult. Sinkholes occur when?