What term describes subterranean, porous, water-holding rocks that provide millions of wells with steady flows of water?

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Groundwater is stored in underground layers that are porous enough to hold water and permeable enough to let it move. Those underground rock formations are called aquifers. They provide water to wells because their pore spaces store the water (porosity) and their connected spaces let water flow (permeability). Recharge from rain and streams maintains the supply, allowing wells to yield water more or less steadily over time. Examples include sandstone or fractured limestone formations, and aquifers can be unconfined (open to the surface at the top) or confined (trapped between impermeable layers and pressurized). The other options aren’t underground water-holding rock formations: volcanoes involve magma, basins are surface depressions, and clathrates are methane hydrates in sediments.