From the Registry Listing:
On the upper slopes and summit ridge of Mount Mansfield Natural Area occur virgin, red spruce-balsam fir forest, extensive alpine tundra, and rare arctic flora not found elsewhere in the northeast. The site includes “The Chin,” the highest peak in Vermont and the 600-foot cliffs of scenic and historic “Smuggler’s Notch.” Designated: 1980. Ownership: State.
Location can be somewhat confusing. The listing says the area is in Chittenden County (thus the "Underhill," above), extending into Lamoille County, which is accurate. The plaque itself is in the Town of Stowe, Lamoille County. You can't get here from Chittenden COunty, unless you're hiking the Long Trail. There also is the Toll Road ($24 for car and 6 occupants) from the Stowe Base Lodge (coordinates below). The Toll Road is a 4-plus mile long, steep dirt road full of hairpins.