Macy is looking for groundwater. she will most likely find it under a mountain in a desert with no plants below a layer of shale in an area with limestone deposits

 in an area with limestone deposits

Explanation:

Limestone is a carbonate sedimentary rock the main composition of the limestone is the calcium carbonate. It can be easily solubilize in water. The groundwater is the water reservoir which is found in the earth geosphere. The  water below the earth crust reaches through the tiny pores inside the underground soil aggregates, and rocks. The water accumulates as a reservoir is freshwater that comes from sources like river, lakes, ponds and river water. The groundwater can be most likely to be found below the limestone deposits as the limestone being a soft material gets easily solubilize by the water and hence, water from surface and subsurface regions makes it's way to the underground.