The word sponge is more appropriate to use than Underground river since an underground river can loose water while an aquifer cannot loose water, unless it is open-air and above ground. then the aquifer would slowly loose water due to evaporation from the sun.
Un nucleótido consiste en una molécula de azúcar (ribosa en el ARN o desoxirribosa en el ADN) unida a un grupo fosfato y una base que contiene nitrógeno. Las bases utilizadas en el ADN son adenina (A), citosina (C), guanina (G) y timina (T).