Because it bears fruit underground.

Land plants, almost all in the ground to flower, the ground fruit, only peanuts are in the ground to flower the ground under the fruit, so people call it drop peanuts.

After the peanut seedling emerges from the ground, after 18 - 25 days, it begins to flower. In the evening, slowly reveal yellow flowers, to the next morning around 7 o'clock, the flowers open, the same day withered. On the fourth day after flowering, its ovary stalk elongates and grows down into the soil, and the fruit ripens in about 50 days.

Fallen peanut (scientific name: Arachis hypogaea Linn.): annual herb of the genus Fallen peanut in the order Rosaceae, family Leguminosae. Roots with abundant rhizomes; stems and branches angled, leaves papery opposite; petiole base clasping, ovate-oblong to obovate, apex obtuse-rounded, both surfaces hairy, margins with eyelashes; vein margins interconnected to form a web; flowers ca. 8 mm; bracts lanceolate; corolla yellow or golden-yellow, flag petals spreading, pteropetalous petals and keel petals separated, oblong or obliquely ovate, styles extending beyond the calyx tube pharynx, pods swollen. pods thick, June-August flowering and fruiting.

References Peanuts_Baidu Encyclopedia