Which sentence best states the main idea of the passage? (The Water Beneath Central Texas)

Many people picture underground water as a hidden lake. In most places, that picture is wrong. Groundwater is usually stored in the tiny spaces between grains of sand or in cracks and holes inside rock. A layer of rock or sediment that holds water and lets it move through is called an aquifer, and one of the most important aquifers in Texas lies under the central part of the state: the Edwards Aquifer.

The Edwards Aquifer is made mostly of limestone. Limestone slowly dissolves in water that is slightly acidic, and rainwater becomes slightly acidic as it picks up carbon dioxide from the air and the soil. Over a very long time, this water has widened cracks in the limestone into channels and caves. The result is rock full of openings, a little like a hard sponge. Geologists call a landscape shaped this way karst.

Water enters the aquifer in an area called the recharge zone, where the limestone is exposed at or near the surface. When it rains there, water flowing in creeks can disappear into cracks and sinkholes in the creek bed and drain down into the rock. Farther to the south and east, the limestone is buried beneath layers of rock that water cannot pass through easily, so the water below is trapped and held under pressure.

That pressure has two important effects. First, in some places it pushes water up out of the ground on its own, forming springs. Comal Springs in New Braunfels and San Marcos Springs in San Marcos are both fed by the Edwards Aquifer, and they supply the clear, cool water of the Comal and San Marcos Rivers. Second, it makes the aquifer a reliable source of water for wells. The Edwards Aquifer is the main source of drinking water for San Antonio, and farms, businesses, and many smaller towns depend on it too.

Because water moves quickly through karst, the aquifer can refill fast after heavy rain. The same feature makes it vulnerable. During a long drought, pumping can lower the water level faster than rain replaces it, and the flow from the springs drops. Pollution is another concern: a chemical spilled in the recharge zone can reach the water supply with little natural filtering along the way.

The springs and underground waters are also home to several species found nowhere else on Earth. One of them, the Texas blind salamander, spends its life in dark underground water and has only tiny, undeveloped eyes. Several of these species are listed as endangered, so managing the aquifer means balancing the needs of a growing population with the survival of these animals. In 1993, the Texas Legislature created the Edwards Aquifer Authority to regulate how much water is pumped from the aquifer.