Glen Helen Nature Preserve, just east of Yellow Springs beside Antioch College, began with a 700-acre gift in 1929 and grew to 1,147 acres, forming part of more than 2,100 protected acres along the Little Miami River with nearby preserves. Rated exceptional for cleanliness and biodiversity and named a National Natural Landmark in 1965, it includes old-growth forest, the Yellow Spring, Pompeii's Pillar, Orators Mound, and extensive trails. Long used for education and research, it also reflects major restoration work and local advocacy, culminating in the Glen Helen Association taking ownership in 2020 and raising funds for reopening and renewal.