Which of the following is an underground layer of gravel, earth or porous rock that contains water?

The correct answer is an aquifer, which is defined as an underground layer of gravel, earth, or porous rock that has the capacity to hold and transmit water. Aquifers are significant in the hydrological cycle and are crucial for water supply in many areas, serving as natural reservoirs from which groundwater can be extracted. This water can be accessed via wells or springs and is often a key source of fresh water for agricultural, industrial, and domestic uses.

Other terms presented in the options refer to different concepts related to water or geology. A swell does not relate to an underground water source; it typically refers to a gradual increase in wave height or a bulging surface. An artesian well is a specific type of well that taps into a confined aquifer under pressure, allowing water to flow to the surface without pumping. While related to aquifers, it does not define the structure itself. A water shelf, often referred to in hydrology, typically signifies an area where water is stored but is not a recognized term for an underground layer containing water. Thus, "aquifer" stands out as the term that accurately captures the definition in question.